What is the option in mail settings "
Send from domain IP addresses and use domain names in SMTP greeting" and how it works?
If the option is selected, Plesk changes the mail server configuration so that the SMTP greeting will contain the name of the domain from which an email message is sent.
This option helps to prevent the sender's IP address from being put into public black lists, such as the Spamhaus or OpenBL lists. This might happen if the mail server host name is used in SMTP greeting for the messages sent from domain IP addresses. Some recipient servers consider such messages as spam."
More details can be found in official documentation.