Interner MAPI Fehler: Informationen über einen oder mehrere Empfänger...

Hilfreich wären vlt. Angaben zur Installation, Hardware, RAM, CPU und wo ist was installiert. Es liest sich wie Verzögerungen im Informationsfluss.

Eine VM wäre plausibel, dann hierfür bereitgestellter RAM, Anzahl Kerne und die installierte Grommunioversion

@Photon I have seen something similar to your issue, after a new VM build and importing user mailboxes using Outlook cached mode and PST import. You haven't said how you imported your users mailboxes, and this also might not be your issue but it is at least one thing you can check.

When I diagnosed the fault that had been reported to me, I found that although the users 'nickname' list (as they were once called) - the recipient history lists - had been built during the import, some of the contacts were missing their email addresses. Any groups that were imported seemed especially susceptible to this as the MAPI error you report is generated during expansion of the group so the mail fails the delivery attempt with a suitable error message, and the message is not delivered to any of the intended groups recipients.

I and/or the users of the mailbox had to go through the users contacts and check they all had email addresses. That particular system only had a couple of users and around 100 contacts so probably was easier to check/fix by hand, so might not be so easy in your use case. Once all the contacts had their correct email addresses added, the issue was then resolved for both individual recipients as well as for groups that contained the incomplete recipient records.

Best Regards

Mark

Try this:
in /etc/gromox/gromox.cfg add the line: outgoing_smtp_url=sendmail://localhost and restart gromox-http and test again.