DNS Error: Domain name not found

So you have an INTERNAL AD user with an exchange account that you get that error when you email them?
What version of Exchange?

Your user problably has the internal domain as a primary e-mail address. You'd have to change that so the primary e-mail address is your external domain name.
You can do that in the user properties-> e-mail addresses for exchange 2003, or in the exchange management console in the properties of the mailbox (exchange 2007 and up)
Hope this helps!

i have just done this but does it take time before the user profile is updated or is there a way i can speed this account up to take changes?

Normally this shouldn't take more than a few minutes. Be sure to let your user restart his/her outlook.

restarted outlook but now notice that the old profile is mixed up with new one- its office 2010 and when i select change it wont let me and remove wont let me

bit of a mess!

i managed to remove the account and rename it but I am now getting the following:

 #5.5.0 smtp;553 Requested action not taken: mailbox name not allowed

Which version of exchange are you running?

2003

Did you run the cleanup agent in exchange to 'free' the old mailbox?

yes did this but no change.
its strange how messages get out but when outside send in get this message ?

thanks for replying - the mailbox became corrupt and i had to start from scratch