That definitely fixed it.
You might think this is "really only copying in the memory", but actually, it generates network traffic which can be snooped and logged even if they are side by side. That could be desirable in some cases, but seems like overkill.
I did consider installing Dovecot as a container, but Turnkey don't offer a Proxmox Dovecot image and I was a little surprised that an imap mailbox hosting service wasn't included in PMG, and I didn't see the sense using additional resources to provide a complete mail service. Besides, I have experience configuring these side by side so I just followed my nose.
It would be nice if this configuration was supported or even provided in PMG.
Thanks heaps for the help.