How to share a mailbox with multiple users?

Question

IMAP

Hello! Faced with the following task - there is a small company that uses one single box on mail.ru. The volume of letters is about 40 GB, about 42 thousand letters. As always, everything you need. They work in terminal mode. If you connect everyone via IMAP MS Outlook, then the cache is duplicated for each user and becomes simply huge for a small server.
At the moment I have installed a trial version of The Bat! made a shared resource and connected this box to everyone, but The Bat! never worked consistently with IMAP, and a huge amount of letters generally drives him crazy. Thunderbird is not able to work with multiple copies from the same email database.
What other software solutions exist for such a mailbox organization?