Since you didn’t bother filling in the bug report template, and (by your own admission) didn’t check for duplicates, nor could keep to a single issue, i took the liberty of moving your thread to General.
Are you sure the SFOS mail app is inserting invalid characters into the subject?
Can you show us the full error message (last lines in your screenshot), and of course copy-paste the troublesome subject line?
I’m currently not home and have limited access to the internet.
I tried two times to respond to an email I received and it came back twice from all recipients. They all had accounts with united (gmx and web.de). I did not check if it’s connected to united and if new mails are also effected.
The only thing I tried is to delete the last lines of the received email. Came back with the same error.
Your screenshot says that the Subject contains invalid characters, which is why the mail was rejected.
It would be nice to see the complete error message.
And to know what invalid characters the subject might contain. Better use desktop mail for that, not your phone (easier to copy-paste).
FWIW, I’ve had troubles with GMX being way too trigger-happy wrt what they think is spam. Unrelated to SFOS.
The error message had the original message attached. The subject startet with:
Re: Aw:�Re: �Re:
If I look at the source code of the email it looks like this:
Subject: Re: Aw: Re: Re:
There was a non breaking space entered. The first one seems to come from an Android app:
–
Diese Nachricht wurde von meinem Android Mobiltelefon mit WEB.DE Mail gesendet.
The second one must have come from the Jolla mail client. Interesting that Web.de rejects its own subject lines… Or does the Mail client change the non breaking space to a different one? There a plenty of different none braking spaces on unicode.
I will try to put this in a bug report when I have more time.