Wenn using wrong email-characters

Problem/Motivation

If I use incorrect characters in the email, the page reports an unknown error.
The emails are not sent.

I tried it with different email addresses, but I couldn't figure out which characters are not allowed.

Here is a selection that caused the error.

"Lothar Matter" lothar.matter@xxx.ch, Anja Mandowa ,

Proposed resolution

Could you please write instructions, about what is allowed?

Remaining tasks

Thank you. L. Matter