[johnklos]

People don't set up email servers on VPN endpoints. That's not how email works.

[chmike]

You don't need a mail server to send mail. One can send mails through a VPN as client.

[johnklos]

Then the spam is coming from the email servers which are used to relay that spam. Headers can be forged, so the source before the spam server can't be trusted as real.