E-Mail (SMTP) error codes
The classic SMTP error code consists of three digits (e.g. 220 mail.domain.tld SMTP ready.). The ESMTP status codes are additions to the classic code and consist of digits devided by dots (5.1.1 User unknown.). Many times you see both types of code combined (e.g. 550 5.1.1 User unknown). Find below a list of the most common codes:
2xx Request ok. Go ahead.
220 Mail service is running (ESMTP ready).
221 2.0.0 Closing connection (Mail received successfully, Goodbye).
250 2.0.0 Message accepted Read more [...]