Any differences between virtual domains?

[Jianping Song]

My mail server has two domains: a.com and b.com.
Now I want control the delivery process between these two virtual 
doamins. That is, i want to tell qmail that mails delivered to
[EMAIL PROTECTED] are forwarded to [EMAIL PROTECTED] while mails
delivered to [EMAIL PROTECTED] forwarded to [EMAIL PROTECTED] 

What can i do, or if it is possible?

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Best regards
Song                                    mailto: [EMAIL PROTECTED]