Can webmail support multiple logins at the same mail account at same time?

By default the it is forbidden to login to the same account twice at the same time (the session is locked to username/password combination)

The only way how to do ensure this is to use a little trick:

  1. Create userA@domain.com and leave his mailbox path on a default value (domain.com\userA)
  2. Create userB@domain.com AND change his mailbox path manually to value domain.com\userA

So you have two users with two username/password BUT they are in fact going to the same mailbox.