Created attachment 148282 [details]
Screenshot of the sources of the messages.

SUMMARY
Every time I try to send an email requesting delivery confirmation the email isn't sent and I receive a notification that says: "Failed to transport message. Server error: Syntax error in parameters or arguments"

STEPS TO REPRODUCE
1. Write a new mail message
2. Select the option to request a delivery confirmation under Options > Request delivery confirmation
3. Look at the outcome

OBSERVED RESULT
Instead of simply sending the mail with the request, I get an error.

EXPECTED RESULT
It should send the email with the request annexed.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon 5.24
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.93.0
Qt Version: 5.15.3

ADDITIONAL INFORMATION
The SMTP account I'm using to send emails is set to store the password and uses the SSL/TLS port 465 with PLAIN authentication. The issue appears also with STARTTLS port 587.
This happens with a @mail.com account and not with a @gmail.com account, but with the latter I still don't receive the confirmations.

I inspected the sources of the two messages, the one I tried to sent with @mail.com (and was never sent) and the one sent with @gmail.com (that was actually sent).
I thought that such "syntax error" could be seen from the source. I see that they are very different.