I cant remember the exact error message but it was something like "unable to establish connection with smtp server".
The original dns settings were the ones the computer found when connecting to Swisscom.
Before the dns was changed (after the mail stopped sending), the mail was always slow to send at my home address sometimes taking up to 60 seconds. i never had this problem with any of my other devices or when using my macbook away from home.
Since changing the dns, the email is going straight away.