Email validation: graylist handling

[vohmar]

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;

[vohmar]

The process:

1. email validation smtp check returns temporary error - email address get grey-listed status
2. new check is scheduled (variable intervals minimum 30 minutes for up to 5 days)
3. when the last check fails - grey-list check failed status is set to the email address
4. force delete status is set to the domains names related to the failed address
5. next daily check resulting with the same temporary error will keep the failed status and will not trigger new grey list handling process

[vohmar]

please add this functionality to our email validator so it can handle the gray list smtp servers properly on smtp level email validation checks