I can’t send emails from the Mail app on my macbook, the following error message appears:

I tried with different settings but didn’t get very far.
So I installed the ‘My Swisscom Assistant’ to help fix the problem. When I click to log in with my Swisscom Login credentials a window opens, but no matter how long I wait, it remains blank without giving any options or whatsoever.

Am I doing anything wrong? How can this problem be fixed?
Also, when browsing on the Swisscom website, I often get the following message from Safari:

What’s wrong?