Change email address format and new domain in SBS 2008

Good day to all, and thank you for any advice you might be able to give on this question!

I have a situation where we have SBS2008 hosting our e-mail, using the domain of (say) A.COM.  We have A.COM externally hosted, and are using the SBS POP3 connector to receive mail, and this is working well.

The company has merged with other businesses, and now we need to change the main domain of the e-mail to B.COM, and at the same time, all users need to change their email address format from (assuming their name was John Smith) to  

The email for B.COM will also be externally hosted, and we can use POP3 Connector to get e-mail for this too.  This aspect of how the email for B.COM is hosted and using the POP3 Connector cannot be changed, it is mandated by agreement of all offices involved in the merge.

The email for B.COM will now be shared among all offices, so this location's Exchange must be able to send mail to folks outside of this location using the B.COM e-mail addresses belonging to other offices.

The company have several mobile users using Outlook Anywhere which we have connected successfully and have a third party certificate for A.COM, and several users have phones synchronised to Exchange also using A.COM.  This location will retain full rights to continue to use A.COM so this method of connection/certificate etc. can easily be maintained if needed.

Now that I hope I have explained background and requirements well enough (any questions please ask!), here is what I would like to know:

In order to achieve all these things, what is the best way of going about doing this?  I have looked through much documentation already, and in fact, whilst I'm loathe to influence anyone's thinking, my idea was to simply create a new accepted domain on the Exchange server for B.COM, then manually enter everyone's new address via the Exchange Console, setting that new e-mail address as their primary.  Then build the appropriate POP3 connectors to download the mail and bring it into Exchange.

I felt that by doing it this way - and "internally" for remote/phone/Outlook Anywhere connections, still using the address would keep things as simple as possible.

Then I wondered - could it really be just that simple?  I'd appreciate thoughts and advice on this - unfortunately, I don't have the chance to test these things first, and as it's a production environment that's quite crucial, I need to get it right from the get-go.

Many thanks in advance for any assistance folks may be able to offer!
Peter ClahsenAsked:
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Larry Struckmeyer MVPCommented:
Try an internet search on this:  "SBS send mail to users with same domain name but not on exchange"  I did and got many hits.  Since you have the pop3 twist in the mix there is not one definitive answer and I haven't the time to compose one.  But you should be able to work it out from that search or similar.
Larry Struckmeyer MVPCommented:
I don't see anything wrong with your plan (except I would not use POP for Exchange mail).  You don't say anything about the other new company's email situation so can I assume they will use Outlook Anywhere against the SBS/Exchange server? Or maybe OWA?    The below is the only hard part of your scenario, and it should be resolved by anywhere if there is not another Exchange server in the mix.

The email for B.COM will now be shared among all offices, so this location's Exchange must be able to send mail to folks outside of this location using the B.COM e-mail addresses belonging to other offices.
Peter ClahsenAuthor Commented:
Thank you for your comments Larry!

To clarify - email for B.COM will be hosted by an external provider, and POP3 mailboxes will be supplied.  This is an aspect that has to be that way, as this is the only situation acceptable to all parties.

The external provider will have POP3 accounts for everyone in the organisation, in all offices.  I am only responsible for one of these offices, and hence the situation I'm asking about :)

On that basis, the Exchange server I am responsible for will only have accounts in the B.COM space for folks inside this office - it will collect incoming mail using the POP3 connector.  My biggest concern is that I need it to work so that when folks inside this office send an e-mail to a B.COM email address for someone in another office, given that the mailbox is not local, Exchange would then relay the message out to the host at the external provider.

It is this very aspect that has me most "concerned" as such - or, to put it another way, it's the aspect I'm least sure about.

On that basis, is my plan still correct, or must I approach this in a different way to have Exchange behave the way we want it to?
Peter ClahsenAuthor Commented:
For those who may come across this thread in the future, Larry hit the nail right on the head - I was searching for the wrong phrases and not getting good results.

His hint on a search phrase led me to which encapsulates the perfect solution for what we needed to achieve.

Many thanks Larry for your help on this one!
Peter ClahsenAuthor Commented:
Including my own reply as this was the result of Larry's assistance in obtaining the answers needed
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