Kubuntu 16.04 with (with a "dog ate"my Plasma Desktop); dual boot with Windows 10. Having a strange, IMO, problem with KMail.

When I try to send an email I get the following error:

Failed to transport message. The server did not accept the sender address "JohnPayne@john-x555lab". Server responded "<JohnPayne@john-x555lab>: "@" or "." expected after "John".

For reasons unknown, the space time continuum may have shifted to a different universe, the out box in Local Folder has created a new (but bogus) SMTP email address composed of my name and the root name (x555-lab) and is trying to use this bogus email to send out bound messages.
Small wonder the server did not accept the bogus email.

My default SMTP server is setup correctly.

Any help would be appreciated.
Thanks
John

Can you check the "default domain" in the account configuration? IIRC, if it's not set, it will default to the machine hostname. Also check your identity settings.