I am asking this here because I could not find a solution anywhere on the web.
KMail can currently receive my email but will not send any (either to mailing lists or to a regular address)
The error it gives me is:
"5.7.1 Rejected due to unmatching envelope and header sender."
How do I fix this?
I just fixed this issue by setting the SMTP Authentication to XOAUTH2 from default PLAIN.
Goto Settings >> Configure Kmail... >> Accounts Switch to "Sending" tab. Select the entry for smtp.gmail.com MOdify and in Advanced tab change "Authentication" option value to XOAUTH2.
I had an email alias inserted in my Identity. Removing that solved the problem for me.
I found the solution!
Actually, the issue is that KMail has a somewhat confusing (at least to me) distinction between identities and email accounts. So my main identity was a gmail one, but I was trying to send (with unified inboxes) an email from a different server altogether.
I found out that this was the culprit after setting up Kmail in another machine. Since it was new I did not import the account from Thunderbird, I just set everything up from scratch, and did not retain a default gmail identity. I did not have any issues in this new install, and therefore I assume this was the issue.
If you have an Office 365 account, make sure that your outgoing server is set as "Microsoft Exchange Server (EWS) transport agent". For that, go to Settings > Configure KMail... > Accounts > Sending. You should see the former in the "Type" column. If you don't, add a new account for outgoing server, and follow the steps in the respective KMail wiki article.
I tried everything from this post to no avail, and this fixed my issue. It looks like I (or KMail automatically) had set up an SMTP outgoing account by mistake, and then no matter what I did, I couldn't send any mail at all. Credit goes to emptythevoid.
