Issue Outgoing emails received from wrong domain

[stevland]

When I look at the header of emails that pass through my server, I often see this sort of thing:

Received: from mail-yt3can01on2121.outbound.protection.outlook.com (HELO
CAN01-YT3-obe.outbound.protection.outlook.com) (40.107.115.121)
by foo.com with (AES256-GCM-SHA384 encrypted) SMTP; 22 Apr 2022 10:03:36 -0700

foo.com is always one of the domains that I host, rather than the domain that I use as my hostname / SMTP server.

foo.com is always different. It seems to pick up a different domain randomly for each email.

I suspect this has something to do with the fact that all of the accounts on my server share one IP address. Does this have something to do with reverse DNS lookup on the receiver's end?

In any event, how can I nail this down to always use my hostname?

 

[stevland]

P.S.
CentOS Linux 7.9.2009 (Core)
Plesk Obsidian 18.0.43
courier-imap 5.1.7-2.centos.7+p18.0.43.0+t220406.1450
psa-qmail 1.03-2.centos.7+p18.0.43.0+t220406.1450