How Do I Access a Shared Mailbox Via the Web Interface?

In case you have a personal exchange mailbox access is very easy:

• Log into Webmail (https://mail.tum.de).
• Then click on the personal symbol at the top right corner and on "Weiteres Postfach öffnen".
• There you can enter the e-mail address of the shared mailbox.

Otherwise you have to use the alias of the shared mailbox in the URL. Form an URL like the following: mail.tum.de/owa/alias@ads.mwn.de/ where you have to replace "alias" accordingly.

Further information you can find in the 

Exchange manual.