[CentOS] Tracking down application sending mail in CentOS 7

I am running CentOS 7 on a server where I have only recently installed postfix. Postfix is configured to only send outgoing mail via an external relay server and test mails created on my server are indeed delivered via this external server.

However, I must have some other application on the server generating mails where the recipient's address is incorrect resulting in being rejected by the external relay server.

Viewing /var/log/maillog I get some information but cannot see which application generated that e-mail, nor the content of it which would likely allow me to see where it comes from.

Can anyone suggest how to track down the app so I can reconfigure the mail address?

Thanks.