outlook using wrong reply address

Hello.

We have an exchange server running that deals with all the internal email
and a pop3 server (hotes externally) for external email.

Each user had 2 accounts in their Outlook account, the external one (set to
be the default) and the internal. The internal email address is their log
in name (eg 'johns') where as the external email is their full name (eg
'john.smith@.....' )

The problem we are having is that SOME when being replied to are coming back
to the 'johns@....' instead of the 'john.smith@....' .

I have made sure that the default account is the external mail server and
that the reply addresses are correct. I have also made sure when sending
emails that the user is using the external email account.

Any ideas what could be going wrong here?

Thanks in advance

Gavin