IF you do not receive the email containing the details promised in the error message
AND
the File column is blank
AND
you only have a single error line relating to webmail
THEN (and only then)
It is likely that there's no configuration error and that all that's happened is that an error condition has been generated and left in the Configurations table for a domain that no longer exists. This is why there's no file.
In this situation it is usually OK to remove the row in question from the Configurations table as long as you do a httpdmng --reconfigure-all afterwards and you get no errors (other than possibly mailman).
Code:
mysql> DELETE FROM Configurations WHERE id='ID-OF-LINE-WITH-ERROR-CONDITION';
I do not understand why httpdmng --reconfigure-all does not remove the error condition all by itself. That's a bit of a mystery to me.
For more details and ideas, take a look at
http://kb.parallels.com/en/111605 but keep in mind that it has been badly written. What it is actually saying is that the error could be a result of any of the situations listed at the start and not that it is definitely a result of all of them.
For the webmail issue, take notice of /usr/local/psa/admin/conf/webmail_horde_bootstrap.conf and check out the includes at the bottom of the file.
The domain you deleted, referenced in the line you remove from the Configurations table (cross-reference with the domains table -- the objectid should not be in use it will have been skipped), should not be listed.
A "long-cut" to doing what I've basically described can be found in item 2 in the Resolutions section in that KB. If you read it, you'll see how it is basically telling you to remove all references for all domains used in building the apache configurations. It also tells you to remove everything from the Configurations table. All these things will be re-built and recreated when you run httpdmng --reconfigure-all
The idea is that any references to removed domains and any mangled configuration files will hopefully get removed also.
WARNING: Messing with the Plesk database tables is dangerous. Do not follow my advice unless you are reasonably sure it is the right thing to do and will work for you. Always make a backup. Be prepared for everything to be screwed up. Do not assume I'm right about anything. OK? My advice worked for me. It might not for you. It might make things worse.
EDIT: p.s. you'll probably find a (webmail) configuration file for the deleted domain in /usr/local/psa/admin/conf/generated
-- you may want to remove that.