diff --git a/composer.json b/composer.json
index d8310a5..a90808d 100644
--- a/composer.json
+++ b/composer.json
@@ -28,7 +28,7 @@
         "netglue/laminas-mail-utils": "^0",
         "netglue/psr-container-postmark": "^1.6.0",
         "psr/cache": "^1.0 || ^2.0.0 || ^3.0.0",
-        "wildbit/postmark-php": "^4.0"
+        "wildbit/postmark-php": "^4.0 || ^6.0"
     },
     "autoload": {
         "psr-4": {
diff --git a/composer.lock b/composer.lock
index 8ef8274..4ad57de 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "b880c50ca6ba5048c5a37614dc93b669",
+    "content-hash": "e82ecbaa74a5c4accdeb4f0c0ab28731",
     "packages": [
         {
             "name": "guzzlehttp/guzzle",
@@ -1688,24 +1688,26 @@
         },
         {
             "name": "wildbit/postmark-php",
-            "version": "v4.0.5",
+            "version": "v6.0.5",
             "source": {
                 "type": "git",
                 "url": "https://github.com/ActiveCampaign/postmark-php.git",
-                "reference": "b71efba061de7cf7e1f853d211b1c5edce4e3c5b"
+                "reference": "ac5354a7591b9b1a9a31281425efc227fb01cbd7"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/ActiveCampaign/postmark-php/zipball/b71efba061de7cf7e1f853d211b1c5edce4e3c5b",
-                "reference": "b71efba061de7cf7e1f853d211b1c5edce4e3c5b",
+                "url": "https://api.github.com/repos/ActiveCampaign/postmark-php/zipball/ac5354a7591b9b1a9a31281425efc227fb01cbd7",
+                "reference": "ac5354a7591b9b1a9a31281425efc227fb01cbd7",
                 "shasum": ""
             },
             "require": {
-                "guzzlehttp/guzzle": "^6.0|^7.0",
-                "php": ">=7.0.0"
+                "guzzlehttp/guzzle": "^7.8",
+                "php": "~8.0 || ~8.1 || ~8.2"
             },
             "require-dev": {
-                "phpunit/phpunit": "^6.0.0"
+                "friendsofphp/php-cs-fixer": "^3.40",
+                "phpstan/phpstan": "^1.10",
+                "phpunit/phpunit": "^9"
             },
             "type": "library",
             "autoload": {
@@ -1720,9 +1722,9 @@
             "description": "The officially supported client for Postmark (http://postmarkapp.com)",
             "support": {
                 "issues": "https://github.com/ActiveCampaign/postmark-php/issues",
-                "source": "https://github.com/ActiveCampaign/postmark-php/tree/v4.0.5"
+                "source": "https://github.com/ActiveCampaign/postmark-php/tree/v6.0.5"
             },
-            "time": "2023-02-03T15:00:17+00:00"
+            "time": "2024-01-09T21:15:01+00:00"
         }
     ],
     "packages-dev": [