How to move users/mail between two forests, keep the e-mail address

In our company, we have two separate domains, with a trust between. We'd like to have all users in one domain, and decommission the other domain.

The larger of the two domains (DomA) has a .com e-mail suffix, the smaller (DomB) has a .ie.
Most users in DomB use Citrix to access apps and resources in DomA, so any DomB user that uses Citrix has a DomA account as well.
Most of these users with two accounts have different account names (e.g. for John Smith, DomA - jsmith, DomB - john).
The DomA domain is W2k8 R2 with Exchange 2010, the DomB domain is Win 2k3 with Exchange 2k3.
When all DomB users are logging in to DomA, DomB will no longer be used.

In the end, we want DomB users logging in to DomA, and keeping their DomB .ie maill suffix. We'd prefer to move people in groups, and not all at once. The existing DomA accounts will have Exchange mailboxes created (they do not exist now). When a user logs in to DomA, we'd like to have their mail delivered to the DomA Exchange server, while maintaining the .ie e-mail suffix.

How do you have a DomB user log in to DomA, keep the same DomB e-mail address and receive e-mail while on the DomA Exchange server, and have the remaining DomB users/mail not affected? Or is it possible? If it is, what would that config look like?

We've considered deleting the DomA accounts, and migrating the DomB accounts over, but it doesn't keep the account naming convention consistent, and doesn't address how the .ie e-mail follows them to the new server.

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Roshan_cConnect With a Mentor Commented:

firstly you need to update the mx record, presently how your mails are routed for both domain.

for domainb .ie create secondary record pointing to domaina (.com)
now in exchange 2010 make entry of .ie as trusted domain
now create alias email ID for the user one by one
for example create alias email id for john in domaina(.com) similar to his present mail id in domainb (.ie)
temporary disable his id in domainb

test sendin mail to his .ie mail id and verify if it is received in domaina

now make copy of profile if user is directly logging into domainb, and login to domaina using the credential domaina
copy the profile and done

move the users one by one make sense
rjramosAuthor Commented:
That sounds straightforward enough. Wouldn't disabling the account in domain b cause a bounce though?
rjramosAuthor Commented:
Alright, close enough for points. Thanks
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