I have just done a fresh install of a new computer with Ubuntu 20.4 LTS and the Kontact PIM suite installed stock standard.

I have imported my Kmail from the old machine and I have copied the SMTP setup for my service provider exactly from my old computer to the new and confirmed with the service provider that the setup is correct. I have checked and re-checked that the setup is exactly the same. My old computer is still working so I can test against that. When I send out an email I get a failed authorisation message before sending fails on the new machine. The same setup on the old machine works perfectly.

What am I missing here?

This is the error message I get:

"E-Mail Sending Failed
Failed to transport message. Server error:
Incorrect authentication data"

The authentication data is confirmed and re-confirmed to be correct. It works on the old machine.

You are lucky to have a error message
Sadly, while the KMail sound powerful, I always gave up and end using Thunderbird.
Numerous time I searched a solution and posted about KMail in this forum without great success.
sometimes it work for a week, sometimes I figure my email never reach my SMTP server but are considered sent by KMail, ...
it is sad.

lexattor wrote:This is the error message I get:

"E-Mail Sending Failed
Failed to transport message. Server error:
Incorrect authentication data"

The authentication data is confirmed and re-confirmed to be correct. It works on the old machine.