diagnose WF Notification mail

In this 12.1 environment I'm trying to do the following:

Other environments have the Workflow Notification mailer setup, mails being sent, etc. etc. But in this environment, no mail is being sent, when setting it up the same way.

The eBS middleware runs on different servers, and they (try to) contact the same SMTP mail server.

I'm thinking the issue is on OS/network level, not inside eBS itself.

There are many variables in eBS, but I'm finding none of them seems to mention the method or call that is actually used to send mails, like the 'mail' command is doing.