Go to https://webmail.au.dk
Enter your AU username: au[AUID]@uni.au.dk, for example au12345@uni.au.dk
Password: Enter the password you normally use on AU's systems.
In addition to your login you will get prompted with two-step authentication. Read more about two-step authentification
It is possible to add an alternative email address to your student email (email alias). You can find a guide for setting up mailalias in webmail here