This might have several reasons. maybe the configuration regarding smtp and pop/imap does not match.
As far as I am aware, the logindata of a user must be equal to the login-data of the corresponding mailserveraccount.
What I am trying to say is:
Emailaccount (on the mailserver) "Name.Number@Mailserver.de" having password “hehe101” should be queried by an egroupwareaccount (on the egroupware system) of the user “Name.Number”, also having password hehe101".
Additionally emailaccounts, that might be included for this user, do not fall under this rule.
Maybe you should check that out^^
Hope I could help