This indicates that the email provider, mail.dk, has changed requirements for their SMTP servers. If you post screenshot of your SMTP server settings for that account, someone here may have suggestions on what to change. We have been seeing such changes all year, as providers tighten security requirements. Some changes have been a simple as requiring full email addresses, or requiring password security.

You're probably right. I have uploaded screenshots of the server settings.

I suggest trying ssl/tls or starttls to see if that does it . and it may require trying different combinations of ports, either 465 or 587 to find the right combination.

None of the combinations ssl/tls, starttls, 465 and 587 worked. But on the starttls the system suddenly suggested 110, and that worked!!!!! Only problem now is that I still can't send mails from my phone (Android). Any suggestions?

I suggest making the same change to your android phone. starttls and 110. I cannot tell you the steps because each app is different.

See https://support.mozilla.org/en-US/questions/1431162