diff --git a/src/Riok.Mapperly/Descriptors/MappingBodyBuilders/RuntimeTargetTypeMappingBodyBuilder.cs b/src/Riok.Mapperly/Descriptors/MappingBodyBuilders/RuntimeTargetTypeMappingBodyBuilder.cs
index cc92fee709..832ecc36dc 100644
--- a/src/Riok.Mapperly/Descriptors/MappingBodyBuilders/RuntimeTargetTypeMappingBodyBuilder.cs
+++ b/src/Riok.Mapperly/Descriptors/MappingBodyBuilders/RuntimeTargetTypeMappingBodyBuilder.cs
@@ -39,7 +39,7 @@ private static IEnumerable<ITypeMapping> GetUserMappingCandidates(MappingBuilder
         foreach (var userMapping in ctx.UserMappings)
         {
             // exclude runtime target type mappings
-            if (userMapping is UserDefinedNewInstanceRuntimeTargetTypeMapping)
+            if (userMapping is UserDefinedNewInstanceRuntimeTargetTypeMapping or UserImplementedExistingTargetMethodMapping)
                 continue;
 
             if (userMapping.CallableByOtherMappings)
diff --git a/src/Riok.Mapperly/Descriptors/MappingCollection.cs b/src/Riok.Mapperly/Descriptors/MappingCollection.cs
index 9bceccae3a..19ec7746b0 100644
--- a/src/Riok.Mapperly/Descriptors/MappingCollection.cs
+++ b/src/Riok.Mapperly/Descriptors/MappingCollection.cs
@@ -66,13 +66,12 @@ public void Add(ITypeMapping mapping)
             _methodMappings.Add(methodMapping);
         }
 
-        if (
-            mapping.CallableByOtherMappings
-            && mapping is UserImplementedExistingTargetMethodMapping existingMapping
-            && FindExistingInstanceMapping(mapping.SourceType, mapping.TargetType) is null
-        )
+        if (mapping.CallableByOtherMappings && mapping is UserImplementedExistingTargetMethodMapping existingMapping)
         {
-            _existingTargetMappings.Add(new TypeMappingKey(mapping), existingMapping);
+            if (FindExistingInstanceMapping(mapping.SourceType, mapping.TargetType) is null)
+            {
+                _existingTargetMappings.Add(new TypeMappingKey(mapping), existingMapping);
+            }
             return;
         }
 
diff --git a/src/Riok.Mapperly/Descriptors/Mappings/UserMappings/UserImplementedExistingTargetMethodMapping.cs b/src/Riok.Mapperly/Descriptors/Mappings/UserMappings/UserImplementedExistingTargetMethodMapping.cs
index a366da607b..2244047b85 100644
--- a/src/Riok.Mapperly/Descriptors/Mappings/UserMappings/UserImplementedExistingTargetMethodMapping.cs
+++ b/src/Riok.Mapperly/Descriptors/Mappings/UserMappings/UserImplementedExistingTargetMethodMapping.cs
@@ -37,7 +37,9 @@ public UserImplementedExistingTargetMethodMapping(
     public bool IsSynthetic => false;
 
     public ExpressionSyntax Build(TypeMappingBuildContext ctx) =>
-        throw new InvalidOperationException($"{nameof(UserDefinedExistingTargetMethodMapping)} does not support {nameof(Build)}");
+        throw new InvalidOperationException(
+            $"{nameof(UserImplementedExistingTargetMethodMapping)} {ctx.Source}, {ctx.ReferenceHandler} does not support {nameof(Build)}"
+        );
 
     public override IEnumerable<StatementSyntax> Build(TypeMappingBuildContext ctx, ExpressionSyntax target)
     {
diff --git a/test/Riok.Mapperly.IntegrationTests/BaseMapperTest.cs b/test/Riok.Mapperly.IntegrationTests/BaseMapperTest.cs
index 0eaa946613..8e547c30f3 100644
--- a/test/Riok.Mapperly.IntegrationTests/BaseMapperTest.cs
+++ b/test/Riok.Mapperly.IntegrationTests/BaseMapperTest.cs
@@ -109,6 +109,7 @@ public static TestObject NewTestObj()
                 ExistingISet = { "1", "2", "3", },
                 ExistingHashSet = { "1", "2", "3", },
                 ExistingSortedSet = { "1", "2", "3", },
+                ExistingList = { "1", "2", "3", },
                 ISet = new HashSet<string> { "1", "2", "3", },
 #if NET5_0_OR_GREATER
                 IReadOnlySet = new HashSet<string> { "1", "2", "3", },
diff --git a/test/Riok.Mapperly.IntegrationTests/Dto/TestObjectDto.cs b/test/Riok.Mapperly.IntegrationTests/Dto/TestObjectDto.cs
index 2e0c554ea2..ea814ea60f 100644
--- a/test/Riok.Mapperly.IntegrationTests/Dto/TestObjectDto.cs
+++ b/test/Riok.Mapperly.IntegrationTests/Dto/TestObjectDto.cs
@@ -79,9 +79,11 @@ public TestObjectDto(int ctorValue, int unknownValue = 10, int ctorValue2 = 100)
 
         public ISet<int> ExistingISet { get; } = new HashSet<int>();
 
-        public HashSet<int> ExistingHashSet { get; } = new HashSet<int>();
+        public HashSet<int> ExistingHashSet { get; } = new();
 
-        public SortedSet<int> ExistingSortedSet { get; } = new SortedSet<int>();
+        public SortedSet<int> ExistingSortedSet { get; } = new();
+
+        public List<int> ExistingList { get; } = new();
 
         public ISet<int> ISet { get; set; } = new HashSet<int>();
 
diff --git a/test/Riok.Mapperly.IntegrationTests/Mapper/StaticTestMapper.cs b/test/Riok.Mapperly.IntegrationTests/Mapper/StaticTestMapper.cs
index 84c41bbf12..ffe5161dd3 100644
--- a/test/Riok.Mapperly.IntegrationTests/Mapper/StaticTestMapper.cs
+++ b/test/Riok.Mapperly.IntegrationTests/Mapper/StaticTestMapper.cs
@@ -38,6 +38,15 @@ public static TestObjectDto MapToDto(TestObject src)
             return target;
         }
 
+        public static void MapExistingList(List<string> src, List<int> dst)
+        {
+            dst.EnsureCapacity(src.Count + dst.Count);
+            foreach (var item in src)
+            {
+                dst.Add(int.Parse(item));
+            }
+        }
+
         // disable obsolete warning, as the obsolete attribute should still be tested.
 #pragma warning disable CS0618
         [MapperIgnore(nameof(TestObjectDto.IgnoredStringValue))]
diff --git a/test/Riok.Mapperly.IntegrationTests/Models/TestObject.cs b/test/Riok.Mapperly.IntegrationTests/Models/TestObject.cs
index c01df23838..86605edc40 100644
--- a/test/Riok.Mapperly.IntegrationTests/Models/TestObject.cs
+++ b/test/Riok.Mapperly.IntegrationTests/Models/TestObject.cs
@@ -77,9 +77,11 @@ public TestObject(int ctorValue, int unknownValue = 10, int ctorValue2 = 100)
 
         public ISet<string> ExistingISet { get; } = new HashSet<string>();
 
-        public HashSet<string> ExistingHashSet { get; } = new HashSet<string>();
+        public HashSet<string> ExistingHashSet { get; } = new();
 
-        public SortedSet<string> ExistingSortedSet { get; } = new SortedSet<string>();
+        public SortedSet<string> ExistingSortedSet { get; } = new();
+
+        public List<string> ExistingList { get; } = new();
 
         public ISet<string> ISet { get; set; } = new HashSet<string>();
 
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/NET_48/MapperTest.RunMappingShouldWork.verified.txt b/test/Riok.Mapperly.IntegrationTests/_snapshots/NET_48/MapperTest.RunMappingShouldWork.verified.txt
index 6f6e920259..d176ba58f4 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/NET_48/MapperTest.RunMappingShouldWork.verified.txt
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/NET_48/MapperTest.RunMappingShouldWork.verified.txt
@@ -56,7 +56,7 @@
     ImmutableArrayValue: null,
     ImmutableQueueValue: [],
     ImmutableStackValue: [],
-    EnumReverseStringValue: 
+    EnumReverseStringValue:
   },
   NullableReadOnlyObjectCollection: [
     {
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.RunMappingShouldWork.verified.txt b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.RunMappingShouldWork.verified.txt
index 90335d9b16..def9404732 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.RunMappingShouldWork.verified.txt
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.RunMappingShouldWork.verified.txt
@@ -124,6 +124,11 @@
     2,
     3
   ],
+  ExistingList: [
+    1,
+    2,
+    3
+  ],
   ISet: [
     1,
     2,
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.SnapshotGeneratedSource.verified.cs b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.SnapshotGeneratedSource.verified.cs
index df5b5c8a04..030736aa6e 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.SnapshotGeneratedSource.verified.cs
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/DeepCloningMapperTest.SnapshotGeneratedSource.verified.cs
@@ -86,6 +86,12 @@ public static partial class DeepCloningMapper
                 target.ExistingSortedSet.Add(item2);
             }
 
+            target.ExistingList.EnsureCapacity(src.ExistingList.Count + target.ExistingList.Count);
+            foreach (var item3 in src.ExistingList)
+            {
+                target.ExistingList.Add(item3);
+            }
+
             target.ISet = global::System.Linq.Enumerable.ToHashSet(src.ISet);
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(src.IReadOnlySet);
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(src.HashSet);
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.RunMappingShouldWork.verified.txt b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.RunMappingShouldWork.verified.txt
index 4d03e0e639..262da34565 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.RunMappingShouldWork.verified.txt
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.RunMappingShouldWork.verified.txt
@@ -140,6 +140,11 @@
     2,
     3
   ],
+  ExistingList: [
+    1,
+    2,
+    3
+  ],
   ISet: [
     1,
     2,
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.SnapshotGeneratedSource.verified.cs b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.SnapshotGeneratedSource.verified.cs
index 6045c2198d..66028394e4 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.SnapshotGeneratedSource.verified.cs
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/MapperTest.SnapshotGeneratedSource.verified.cs
@@ -127,6 +127,12 @@ public partial int ParseableInt(string value)
                 target.ExistingSortedSet.Add(ParseableInt(item2));
             }
 
+            target.ExistingList.EnsureCapacity(testObject.ExistingList.Count + target.ExistingList.Count);
+            foreach (var item3 in testObject.ExistingList)
+            {
+                target.ExistingList.Add(ParseableInt(item3));
+            }
+
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(testObject.ISet, x => ParseableInt(x)));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(testObject.IReadOnlySet, x => ParseableInt(x)));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(testObject.HashSet, x => ParseableInt(x)));
@@ -211,6 +217,12 @@ public partial int ParseableInt(string value)
                 target.ExistingSortedSet.Add(item2.ToString());
             }
 
+            target.ExistingList.EnsureCapacity(dto.ExistingList.Count + target.ExistingList.Count);
+            foreach (var item3 in dto.ExistingList)
+            {
+                target.ExistingList.Add(item3.ToString());
+            }
+
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(dto.ISet, x => x.ToString()));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(dto.IReadOnlySet, x => x.ToString()));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(dto.HashSet, x => x.ToString()));
@@ -296,6 +308,12 @@ public partial void UpdateDto(global::Riok.Mapperly.IntegrationTests.Models.Test
                 target.ExistingSortedSet.Add(ParseableInt(item2));
             }
 
+            target.ExistingList.EnsureCapacity(source.ExistingList.Count + target.ExistingList.Count);
+            foreach (var item3 in source.ExistingList)
+            {
+                target.ExistingList.Add(ParseableInt(item3));
+            }
+
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(source.ISet, x => ParseableInt(x)));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(source.IReadOnlySet, x => ParseableInt(x)));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(source.HashSet, x => ParseableInt(x)));
@@ -444,4 +462,4 @@ private string MapToString1(global::Riok.Mapperly.IntegrationTests.Dto.TestEnumD
             return target;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunExtensionMappingShouldWork.verified.txt b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunExtensionMappingShouldWork.verified.txt
index d0a7271d51..f8d7742c71 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunExtensionMappingShouldWork.verified.txt
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunExtensionMappingShouldWork.verified.txt
@@ -135,6 +135,11 @@
     2,
     3
   ],
+  ExistingList: [
+    1,
+    2,
+    3
+  ],
   ISet: [
     1,
     2,
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunMappingShouldWork.verified.txt b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunMappingShouldWork.verified.txt
index 5ade304416..b764ab8d8d 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunMappingShouldWork.verified.txt
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.RunMappingShouldWork.verified.txt
@@ -140,6 +140,11 @@
     2,
     3
   ],
+  ExistingList: [
+    1,
+    2,
+    3
+  ],
   ISet: [
     1,
     2,
diff --git a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.SnapshotGeneratedSource.verified.cs b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.SnapshotGeneratedSource.verified.cs
index 699997e00c..07cb72804a 100644
--- a/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.SnapshotGeneratedSource.verified.cs
+++ b/test/Riok.Mapperly.IntegrationTests/_snapshots/Roslyn_4_5/StaticMapperTest.SnapshotGeneratedSource.verified.cs
@@ -124,6 +124,7 @@ public static partial int ParseableInt(string value)
                 target.ExistingSortedSet.Add(ParseableInt(item2));
             }
 
+            MapExistingList(src.ExistingList, target.ExistingList);
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(src.ISet, x => ParseableInt(x)));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(src.IReadOnlySet, x => ParseableInt(x)));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(src.HashSet, x => ParseableInt(x)));
@@ -222,6 +223,7 @@ public static partial int ParseableInt(string value)
                 target.ExistingSortedSet.Add(ParseableInt(item2));
             }
 
+            MapExistingList(testObject.ExistingList, target.ExistingList);
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(testObject.ISet, x => ParseableInt(x)));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(testObject.IReadOnlySet, x => ParseableInt(x)));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(testObject.HashSet, x => ParseableInt(x)));
@@ -306,6 +308,12 @@ public static partial int ParseableInt(string value)
                 target.ExistingSortedSet.Add(item2.ToString());
             }
 
+            target.ExistingList.EnsureCapacity(dto.ExistingList.Count + target.ExistingList.Count);
+            foreach (var item3 in dto.ExistingList)
+            {
+                target.ExistingList.Add(item3.ToString());
+            }
+
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(dto.ISet, x => x.ToString()));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(dto.IReadOnlySet, x => x.ToString()));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(dto.HashSet, x => x.ToString()));
@@ -391,6 +399,7 @@ public static partial void UpdateDto(global::Riok.Mapperly.IntegrationTests.Mode
                 target.ExistingSortedSet.Add(ParseableInt(item2));
             }
 
+            MapExistingList(source.ExistingList, target.ExistingList);
             target.ISet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(source.ISet, x => ParseableInt(x)));
             target.IReadOnlySet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(source.IReadOnlySet, x => ParseableInt(x)));
             target.HashSet = global::System.Linq.Enumerable.ToHashSet(global::System.Linq.Enumerable.Select(source.HashSet, x => ParseableInt(x)));