Have you copy&paste the address or did you type it by hand?
On some websites, some “optimisations” make strange changes to texts such as replacing hyphens (-) with minus (−) or ndash (–). Same is true for dots and periods.
I had copied the email address manually, which caused the error. But after clicking the email link directly on the website, it worked fine and was accepted without any issue.
This solution brought me to another solution for my problem: an email address that was saved in the address book of the phone with three empty spaces at the end, produced havoc in K9: it showed the receiver name TWO times, and the emails disappeared when sending or saving them. Deleting the three empty spaces at the end of that saved email address solved the matter.