Exchange 2007 to forward emails to Exchange 2010

Hello Experts!

Here is my situation.

We have purchased a company which has an exchange 2007 server.
Our company has Exchange 2010.
We have no plans on going to the physical location so all has to be done remotely.
Thus there cannot be any interuptions to the emails they are receiving (user@abc.com)

This is what I did.

initally created contacts for all the users at abc.com
This was fine but now we need to be all on the same server.
So I created an email on our exchange server for each user (user@xyz.com) , removed the contact info and added the smtp address user@abc.com into the xyz.com account.

On the Exchange 2007 abc.com - created contacts for all accounts (xyz.com) and did a forward from the abc.com Exchange to xyz.com

So when sending an email from the axyz.com Outlook (internal) it will send to the xyz.com account.  If external emails are sent to the abc.com account it will be  in abc.com and forwarded to xyz.com account as well.

For whatever reason emails coming from 3rd party emails are being received by abc.com but are not forwarded to the xyz.com in a timely fashion or not at all.

I have verified the Spam filter and it is not the issue.  What can be causing this issue?

Is there a better way to migrate these users from on server to another without interruption?

Thanks in advance.