Windows XP Professional SP2, Outlook 2003 SP2 connected to Exchange 2003 SP2

I can send a new email to an address but get the following message when
replying. No attachments and it was occuring only occasionally but now on
all.
The following recipient(s) could not be reached:

'target email address' on 5/2/2006 1:02 PM
550 not local host aol.com, not a gateway

Thanks for any help.

Marc Miller