[imp] virtual domains

[ccx]

I'm having problems with Imp and virtual domains.
I'm using imap on localhost (qmail, vpopmail, courier-imap) for horde auth. 
When client authentificates, all goes fine, but when they try to reach their 
email, they have to write their login in form username at virtualdomain.com, 
because according to logs horde/imp strips the @virtualdomain.com and tries 
authentificate as if it was non-virtual user. For non-virtual users that 
don't use @domain suffix to their login it works fine.