How to synchronize mail on three PCs connected via POP3?

Question

Email

Good afternoon everyone!
The essence of the problem: there is one mail address that was connected via the POP3 protocol several years ago and very quickly grew to the size of 25-30GB (several thousand letters, and they are mostly heavy)
Then another person appeared who needed full access to this mail; 30GB to synchronize via IMAP we thought a lot, long and dreary, googling, made synchronization based on a hidden copy of ourselves and several rules in The Bat!
Ok, everything works, no one complains, in the meantime, mail grows to 50GB
Now there is a third person who also needs access to this mail with updated data. They did it through the "blind copy", but it's a crutch, and I don't want to do that, 50GB in IMAP (mail server - mail.ru) - I don't know, we're afraid to synchronize for a week
Actually, the question is: can anyone come across a similar task? Maybe there are specialized solutions? How can you synchronize three PCs without IMAP and hemorrhoids?
The option to clean the box is not suitable, there is a lot of archived data that is used every day

Answer 1

The main thing is not clear from the given description: All mail is stored on the server or at the 1st client?
1. If the mail is on the server, then many mail clients allow you to set up a partial download by imap. I can not imagine a scenario when all 50GB can be required at once.
2. Most client programs allow you to make a backup copy of data, with subsequent restoration on another PC.

Answer 2

copy the whole bat folder?