I've set up KDE Neon 5.23 on an old Dell laptop, and in general it runs well, but I cannot seem to get KMail to send messages. My ISP is Cincinnati Bell, which provides an imap client, and I have two email addresses with them, one for family and friends, and one for the outside world. But when I try to send an email from either one, I get the following message:

E-mail Sending Failed
Failed to transport message. Server error
5.7.1 {P9} Permission denied you cannot
change your from address

I used the autodetect function to set up the accounts. Can someone suggest what I might be missing?
Thanks in advance.

Your mail provider won't allow you to change the From: address, you're probably sending from one identity but logging in with a different account.

Check your identities in Kmail settings and make sure you login with the correct email account.

Cheers.