@McSwizzleStix - I'm not sure there is one end-all, be-all fix. If you're having similar queue issues, it would likely be best to create a new thread with as much detail as you can provide so we can check it out!
In my case it resolved itself. I reset the Exim configuration again, entered the problematic relay as a smart host, and waited a few days until the contract period for that VPS expired before opening another VPS and migrating (with no hope that it would be resolved).
And during those days of waiting, messages progressively began to arrive from forward email accounts.
And during those days of waiting, messages progressively began to arrive from forward email accounts.