Email validator has smtp check that enable quering email server for the existence of a specific mailbox. Some email servers have graylist protection that denies the first request.

Need to add recheck feature to the validator

• if smtp server responds with 4xx reply codes ("please call back later"), as defined in the Simple Mail Transfer Protocol (SMTP) (https://datatracker.ietf.org/doc/html/rfc5321#section-4.2.1). Apparently the error code and message are not standardised. Codes starting with 4 indicate temporary problem and thus the action should be queued for retrying
• how many times and how quickly to retry? When a mail server is greylisted, the duration of time between the initial delay and the retransmission is variable; the greylisting server has no control or visibility of the delay. SMTP says the retry interval should be at least 30 minutes, while the give-up time needs to be at least 4–5 days;