email – Webmail service with multi user access

I am searching for a solution for our business mail accounts.

In specific we have a few mail (imap) addresses which I want to make usable for our staff. The problem is that I want to create user accounts for every staff member and then assign mail accounts to them.

So in the end for example user 1,2,3 has access to mail address A. But user 3 also has access to mail address B.

It could be self hosted but I would prefer a SaaS solution, free or paid.

Thank you
Daniel