The authentication of our webmail (https://owa.ist.ac.at/) has been changed to use our Shibboleth SSO (as most other applications), so it looks like:
If you are on Campus/connected via VPN with an IST device, you should be logged in automatically, as well as if you already logged in somewhere else via SSO.
Because of this change, you can now also enable multi-factor authentication for webmail, which is not only strongly suggested for your IST account, but also for any other account you have (e.g. Google, Facebook, Twitter, Amazon,..). For enabling at IST, please see: MFA Authentication