How do different outgoing mail modes under Tools & Settings > Mail Server Settings work?
Send from domain IP addresses
By default, mail from each domain is sent from the domain's IP address. The host name used in the SMTP greeting is the Plesk server host name specified in Tools & Settings > Server Settings. Selecting this option may result in mail sent from some or all domains being marked as spam if the Plesk server host name fails to resolve properly, or if the domain's IP is different from the one to which the Plesk server host name resolves.
This option works best if it is required to have a single IP address on the Plesk server.
Send from domain IP addresses and use domain names in SMTP greeting
If selected, Plesk changes the mail server configuration so that the SMTP greeting contains the name of the domain from which the email message is sent.
Warning: Selecting this option may result in mail sent from some or all domains being marked as spam if the destination mail server uses cbl and more than one domain on the Plesk server uses the same IP address.
In addition, selecting this option on Plesk servers hosting a large (more than 100) number of domains will likely result in significantly increased server load.
This option works best if there is allocated a dedicated IP address to every domain hosted on the Plesk server, and the number of domains hosted on the server is not very large.
Send from the specified IP address
In case if it is required to use certain IPv4 and IPv6 addresses for all outgoing mail. Sending all mail from the specified address might be useful, for example, if the IP address of the mail server was added to a public black list, such as the Spamhaus or OpenBL lists.
If None is selected, outgoing mail will not be sent.