[imp] IMAP and Fetchmail HowTo

[Alex May]

Can anyone help me understand (or point me in the direction of the right things to read) how to setup horde webmail/imp for my 
setup...
My server hosting horde collects mail from a number of mailboxes using fetchmail and then serves them using courier IMAP. This works 
fine. To send mail I use external SMTP servers which require authorisation.
I have managed to set up horde to log in using my IMAP username and log in, but how do I set it up to send via the external SMTP 
servers. The main problem I see is that I can't find anywhere to add the domain name for the email address.
To make the login work, I can't add the at sign and the domain name to the username for the IMAP server.
I thought that I could set up imp to ignore the at symbol and the domain name when taking the username to log in to the imap server 
using hordeauth true, but that doesn't seem to work. I put that in backends.local.php.
I wonder if using vhosts might help, but I can't find any documentation on how to set that up.
Help!!
Thanks in advance.
Alex