Bounce email address being corrupted in campaign emails

I’ve managed to set up a campaign and send a test email. However, when I look at the headers for that email, it seems that the Return-Path does not match the bounce email address I set up in SuiteCRM.

In particular, “bounce @” is being changed to “srs0=prvbgsuiou=lj=zx48vieusjcmwjxn @” (email addresses edited for spambots).

Surely this cannot be right?

SuiteCRM Version 7.10.7
Sugar Version 6.5.25 (Build 344)

There might be some base64 encoding (or similar) in place. Have you tried checking this through an actual email client, instead of looking directly at the headers?