Exchange server, cannot send to outside of domain only one user

Hello all experts :-)
I'm stuck against a problem I've never seen and cannot resolve on my own.
Since changing email address details in a users (SBS2k3) properties ( I added another email address which didn't work and then I put it back to the original setup the user's email address tab in AD Users etc) is unable to send mail outside of domain this is even with OWA, get bounce back immediately.
Your message did not reach some or all of the intended recipients.The following recipient(s) could not be reached: on 21/04/2010 16:07
  There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
  < #5.5.0 smtp;511 sorry, can't find a valid MX for sender domain (#5.1.1 - chkuser)>
sending_domain is a variable for the actual domain.
I've checked all the users' properties against the other users and nothing is different.
Reverse DNS lookup etc. is OK I've used other tools at and read through all the other posts here at EE. I've checked with my ISP it's not a DNS lookup or MX record issue, the problem is Exchange is not sending out the messages.
I've worked around it by connecting the client to a pop account pointing to the domain but this is not a long term solution. For me this would be a 1000 points on the level of difficulty I hope you can resolve this for me for 500 points.
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What exactly did you change? you could always delete exchange attributes from that user then add them again in AD. I suggest backing up his mailbox before doing this. If there are lots of emails/contacts you could always merge mailboxes afterward.
Jamie McKillopIT ManagerCommented:

Check the defualt SMTP address on the account again. Make sure your didn't make a typo when you put it back. The error you are receiving is from the recipients's server. It indicates there is no MX record for the domain of the user. This tells me that the defualt SMTP address on the account has a typo.


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