Plesk's online guide describes the "Outgoing mail mode" option and its three settings:
The first setting is shown enabled in the circled in red section in the screen shot below:
We recommend you use the first or third option in virtually all cases.
The second option, Send from domain IP addresses and use domain names in SMTP greeting can lead to reputational and CBL issues and be used with extreme care.
Many Plesk installations are unable to bind to different IP addresses to send email on behalf of each hosted domain. This is especially true if the web server uses a single IP address to host multiple domains by "virtual host" - there are no other addresses to bind to, so, it's using the same IP address all the time - which is the problem here.
You should only use the Send from domain IP addresses and use domain names in SMTP greeting if you're certain your installation can successfully bind to specific outbound IP addresses, and each IP address will only be used for at most 2-3 different domains.