Mail clients load thousands of old emails after switching a domain from one hosting to another and back - what to do?

Question

Mail server

Recently, due to problems with Rusonics, I had to urgently switch the domain of the site with mail to another hosting, after the restoration of working capacity they switched back, but a problem arose:
mail clients (in particular The Bat, Microsoft Outlook) began to download letters in a new way in a few years - probably from -for switching the domain to another hosting? or because of setting up email clients via POP3?
Maybe someone faced such a problem?

Answer 1

When migrating mail from one mail server to another mail server, the format and UIDL values ​​had to be synchronized. The mail client remembers the UIDL of received messages if the UIDL has changed - from this point of view, this is a new letter.

Answer 2

The message status "Received / Not received" is usually stored on the mail server, and not in the letter, but in an index file or database. Therefore, when transferring mail to another server, you must transfer this information separately.
And yet, I recommend paying attention, some users of MS Outlook may have the "Automatically send a read report" function enabled when receiving a letter. Those. a user who receives several thousand new emails and clicks "Mark folder as read" will send this report to everyone who has ever asked for it. I got burned on this once.