Can't Recieve External Mail to some users in exchange 2003 org after installing exchange 2010

hi guys,

yesterday i installed exchange 2010 in our organization, all appeared to go well but there are a few users who can't recieve external mail, it keeps bouncing with the following error......

Delivery to the following recipient failed permanently:


Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 5.1.1 <***>: Recipient address rejected: undeliverable address: host[] said: 550 5.7.1 Unable to relay for *** (in reply to RCPT TO command) (state 14).

Any ideas why this would be happening for some users and not others?

btw our exchange server doesn't face the internet directly, it recieves mail from a sophos email appliance, which is synced with AD to only accept for email addresses in our company.... but then the actual exchange server denies it.... :(

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Corp_ITAuthor Commented:
Sorry Guys, Additional, The Users who can't recieve mail from external sources cannot send it to external sources either.
Corp_ITAuthor Commented:
And some more info, i spoke too soon :) all users can send to external sources :P
Corp_ITAuthor Commented:
for those that are interested.... this sorted itself out..... god knows why or how.... but it did. cheers.

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