[Exim] How is the Message-ID generated?

Greetings! Newbie exim user here. Still struggling to send email to AOL
accounts, lol

Anyhow, I have a few domains but just one MX. I can't figure out why my
Message-ID uses one domain over another. To top it off my wifes computer has
a different @<name> than my machines.

Thanks,

-ethan