Switching an MX host from using relaying to using virtual alias domains?

[Michael Durket]

I have a host (A) acting as an MX for host (B). Currently I do this via 
relay_domains and relay_recipient_maps. Is it possible to do this instead via 
virtual_domains and virtual_aliases? Or would a virtual_alias map entry on MX 
host A to forward user@B to user@B not work right (because the right-hand-side 
user@B would generate an MX lookup for B which would loop right back to host A)?