Hello Scarab,
Do you have anything in IMAP and/or POP logs that looks like: "message could not be found" ?
In POP3 logs we have tons of:
RETR failed to FeedMessage for message 4 (UID: 15540) ERR: The group file F:\SmarterMail\Domains\xxx.ch\Users\gilles\Mail\Inbox\2020_5_27.grp is corrupt or a message could not be found in the header..
In IMAP:
[2020.06.11] 00:58:29.347 [188.x.255.x][37227440] Failed to fetch data item 17047 from GRP file for sabxxxxnr@saxxxna.ch in Inbox.
We're having customers reporting the same "This message is no longer on the server. " and our imap/pop3 logs are full of this.
Kind regards,
Sébastien
Sébastien Riccio
System & Network Admin
https://swisscenter.com