550 5.1.1 Recipient address rejected: User unknown

I have a user on Exchange 2007 server, tried adding an additional SMTP address to their mailbox. I e-mail it just fine internally. Externally like from Gmail, I get an error back from the mail server and the final code of "550 5.1.1 Recipient address rejected: User unknown".
Does it take time to sync the new SMTP address?
I tried it with a mailbox on Exchange 2010 and it worked instantly.
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Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
It is supposed to work fine unless you are using a third party system for anti-virus/anti-spam where they might be synchronizing their system with your AD (seen this quite a lot lately).

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garryshapeAuthor Commented:
Yeah there's Symantec Messaging Gateway, I'm not how to get it to sync.
Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Refer to this document, I cannot help as I am not familiar with Symantec Messaging Gateway:

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garryshapeAuthor Commented:
I guess the mailbox I tested with and worked instantly was because apparently an audit of that email shows it didn't flow through Symantec.
garryshapeAuthor Commented:
Interestingly I can find the user based on their address in Symantec, but still get delivery error from outside.
garryshapeAuthor Commented:
This mailbox had been restored a while back and I think they had the address I'm trying to send to now as their only address, which was different than their SamAccountName. So like BThompson@domain.com as their e-mail, instead of BThomson@domain.com.  

Looking at Active Directory, it appears their "mailnickname"attribute is their samaccountname, while their desired mail address is the smtp address with "2" in it.
I wonder if these have to match for it to work.
can u paste the full NDR
garryshapeAuthor Commented:
The problem was with Symantec Messaging Gateway, cache issues with the Windows DC and another setting that affected the other mailbox related to address alias translation
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