@john.combs Sorry for the long delay on this topic. I digged a bit deeper and found in the logs of Mattermost the issue for this problem:
Mattermost seems to send all emails in parallel connections to the smtp server. it does not group and there seems to be no cue for sending emails either one by one or in configurable number groups.
My Server is allowing only 3 parallel connections and there it gives the bottleneck and the issue.
Unfortunately Mattermost doesnt retry after unsuccessful sending of Invitations or notifications @all/@channel etc.
Any hints?
Mattermost 10.5.1 in Containerstation
All the best and Thank You
Klaus