The email addresses used do not contain any special characters, such as accents or umlauts.This cannot be the reason.In addition, both variants were tested as described in the documentation.
"Safe support"
support@safe.com
See also https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Transformers/Transformers/emailer.htm
A quick google shows that it might be related to accented characters in the email address (and assuming you are from Germany that's actually rather likely).That would mean they would get the same error if you try and send email to such an address through other means than the Emailer transformer and that in turn would mean it's not really an FME issue.
There appear to be fixes available for a mailserver to support UTF8 characterset in email addresses.From what I can gather it's mainly Postfix that has this issue and there's an upgrade package called SMTPUTF8 available for it.