[imp] One user has error on SMTP from any webmail menu option

[francis picabia]

There is one user for which any action produces only this sort of message
in the main window of Horde/IMP:

A fatal error has occurred
Failed to add recipient: johndoe at localhost [SMTP: Invalid response
code received from server (code: 550, response: 5.1.8
<johndoe at localhost>: Sender address rejected: Domain not found)]
Details have been logged for the administrator.

In the log it shows:

2012-09-21T09:47:47-03:00 EMERG: HORDE [horde] Failed to add
recipient: johndoe at localhost [SMTP: Invalid response code received
from server (code: 550, response: 5.1.8 <johndoe at localhost>: Sender
address rejected: Domain not found)] [pid 5257 on line 1647 of
"/usr/share/pear/Horde/Mime/Part.php"]

This is with IMP 5.  While I've changed the user to johndoe, the SMTP
is actually shown as localhost.

Usually with these problems I find the user has attempted some
preference which breaks their
set up, and I can remove it from the table horde_prefs.   I have
completely removed everything for that
user from this table and no difference.

I'm not sure what table would contain the bad SMTP setting or whatever
it is which is wedged.

None of my backends.local.php contain localhost, and none of the other
5000 users have encountered this error, so
I'd think it is something in their settings on the DB.