I’m indicating a resolution here. The issue turned out to be network related. The design is as follows:
docker/zammad <—> firewall <—> internet
The service presented by zammad is hosted publicly through a VIP on the firewall. I.o.w. visitors will access a public IP on the firewall (via a public DNS hostname) and then be forwarded on to the Zammad instance which is internal.
When accessing certain functions on Zammad (eg. the verification), traffic flows from Zammad out the firewall and then back in again to the mail server.
This requires special support in the firewall (called hair-pinning) or you need to access the mail server on its internal/private IP address.
I’ve used the 2nd option and this resolves the verification issue.
Thanks for the assistance, Robby