Mailbox creation in Exchange 5.5

Hello.

I have an Exchange 5.5 server. After the creation of a new account (user_A) in my NT domain, the Exchange Server automatically pops up and asks me to create the mailbox for the new user.

I host two domains (domain1.com and domain2.com) on the same server.

Is it possibly for my new user_A to have 2 email addresses:

user_A@domain1.com
and
user_A@domain2.com

I thought to create a distribution list user_A@domain2.com and redirect the email to user_A@domain1.com in order to overcome the problem but the user wants a mailbox in order to perform replies having his mailaddress shown in the From: field.

Thank you in advance.