A summary of the communication exchanged between two mail servers as part of an email message delivery appears in the image below.
The specific acknowledgment our system is looking for, is the response code “250” between stage 10 (on right) to 11 (on left) – highlighted in YELLOW
You can find the full explanation of the SMTP protocol here.
Note that the sample below is for a non-secure connection, if the connection would have been an encrypted Secure SMTP, as part of the session, there would be a preliminary authentication stage (which handles the TLS creation – AUTH with codes 220, 250, 334 and 235).
