At least on version 5.0.10+210723 you can have jori.mäntysalo@somewhere.invalid as a participant address. This is almost always an error, Mäntysalo is written as mantysalo (or maybe maentysalo). Should we have a setting for disabling unicode characters in email addresses?

It seems that some systems behave badly and after trying to sent email having ä or é etc. also following mails are rejected. I'm not sure about this yet.

Please submit a bug report.

A bug report or a feature request? It is possible to have ä at email address, see en.wikipedia.org/wiki/International_email , but it seems to be not used in real life.

If those are actually valid characters for an email address, maybe it should be a feature request for an "allow/disallow" configuration setting.

I am not an expert of the area and have not read newer RFCs. However, I opened bugs.limesurvey.org/view.php?id=17518