We've retired Pegasus since months and working only with Thunderbird IMAP Clients against a local Mercury IMAP Server.
Generally Thunderbird and Mercury cooperate great and all 20 users are happy with Thunderbird.
But from time to time we experience that mails which have been moved into another mail folder (under Thunderbird) reappear in the inbox after a few hours.

When moving a mail, it will be copied into the other mail folder (also physically into the Mercury target mail folder at the server drive) and normally marked as "to be deleted" within the inbox. At this moment the mail disappears from the inbox and is not longer to be seen. (With other client programs like e.g. Roundcube you could still see this "marked as deleted" mail in light grey)
Further in Thunderbird account settings "Expunge inbox on closing" is activated which normally should remove any mails designated for final deletion.

But we do not supervise whether Mercury is really deleting those mails or not. Why they reappear from time to time? Has anybody an idea?

Hi Joerg. I was passing by and wanted to say Hi! Regrettably, I can't help with your question. I am almost 5 years retired now and have forgotten more than I remember about Mercury.

The best way to be sure what is happening is probably to temporarily turn on session logging for MercuryI. It will produce a lot of log data, but every command and response will be included.

@Brian: Hi Brian. Almost 5 years retired . Time is running. Hope you're doing well.

@Rolf: Yes, I switched on the session logging. But it's difficult to search and find for the important lines in such a huge amount of session log files.
Our 20 users do not have only one single user mail account each. Some guys have up to 12 different mail accounts, which are all incorporated into their Thunderbird client. I took care that all mail accounts do not fetch their mails always at the same time to avoid stressing MercuryI. Fortunately this could be adjusted for each single mail account within Thunderbird. Nevertheless there is a lot traffic.
Do you have a simple reference how a command to an IMAP server should looks like for:

• Moving a mail
• Deleting a mail

and how the IMAP server should react or ack to? Maybe then I could search for these commands and the reaction of MercuryI.

edit:

I found this legacy post on our forum:Mercury looses the "\deleted" flag

In order to compare it with our configuration:

• server and clients are in the same LAN. Different clients (like mine) have static IP addresses, other client will get always the same IP addresses due to regular DHCP lease renewal
• normally we do not mix up the mail access (IMAP / POP3). All Thunderbird client access via IMAP
• we always exit Thunderbird in the evening before shutting down the computer. That's why we normally should not have any "broken connection"

But what I can see in the MercuryI status window at the server: Although Thunderbird is opened the whole working day, the status window is showing a logged-in user not permanently. Often the user reappears only when any IMAP operations take place. Don't know whether Thunderbird or other IMAP clients go into any stand-by what causes to a connection loss. But these are my thoughts only. I'm not an IMAP traffic specialist.