How to connect to one mailbox on your server from several mail clients at the same time?

Question

Email

Hello.
I ask to help with such problem:
There is a mail server (iredmail). It has several mailboxes. And there are several employees of the company who must simultaneously monitor all these boxes. Employees use the thunderbird mail client. But everyone periodically gets the error "There was a disconnect from the 'server name'. The server may not be working, or there are problems in the network."
The problem manifests itself when one employee already has a mailbox open in the mail client and the other tries to open it too. But it may be that two people have the same mailbox open, and when the third one opens the mail client with this mailbox, it gives this error.
How to make it possible to monitor one mailbox from any number of mail clients?

Answer 1

1) IMAP
2) server settings
3) Outlook desktop / Essential PIM for error reproducibility

Answer 2

We connect via IMAP, port 143.
I looked at the server settings. Didn't find anything related to concurrent connections. Although there are many settings ..
I want to keep the client thunderbird, because not all Windows. But to check if there will be an error on another client, let's try.