We almost exclusively use Thunderbird as IMAP client here. It works very well and fast with the Mercury IMAP server.
However, we noticed one thing ... In TB we always move edited/read emails to other mail subfolders, so that the inbox folder remains clear.
But if you switch back to Pmail in between, there is a completely different, higher number of mails in the inbox folder. I.e. there are sometimes quite a lot of mails still in the inbox folder, which have long been moved to other subfolders in TB. The moved mails also arrive in the subfolders and are also visible there in Pmail.

Moving mails is also not accompanied by any error message. A consistency check of the individual mail folders in Pmail also shows that everything is fine.

Furthermore, we can also access our mailboxes using Roundcube IMAP web client. There I can at least see that the affected emails have already been marked as "deleted" (disabled). So to me it looks like mails are copied to the subfolders, but then only marked as "deleted" in the inbox folder.

Is there a "timer" somewhere when mails marked as "deleted" are finally deleted?

Just found it by myself. In TB account settings > server settings > message memory, you have to activate the option "clearing, when leaving the inbox (expunge)" (translated from german TB). Then all mails, dedicated for deleting, will not only be marked as deleted but removed. And then the number of new mails at TB matches the number of new mails in Pmail.