If i use copy and paste to insert text into an email and then i send it to an opensuse mailing list, it gets rejected.   This is the message i get from them.
I'm sorry to inform you that your message could not be delivered to the list. Your mail was rejected because it matched a rule set up by thelist administrator.
This list does not allow:
* Attachments
* Mails from other lists
* HTML mails (also multipart text/html!)

My email does not have attachments, its not a mail from another list so by default it must be changing the email to either an "HTML" or "multipart text/html" email

Reproducible: Always

Steps to Reproduce:
1. Compose email
2. Select text from anywhere
3. Paste
4. Send

Actual Results:  
Email is rejected - see details section for rejection message

Expected Results:  
no rejection and the email appearing in the list

I've had emails rejected when copying and pasting from CLI, the same email, webpages and other emails.
Its been happening to me for a long time (years) and i've just got fed up working around it.  It took me a long while to realise why it was happening.