xp_sendmail failed with mail error 0x800040005

[IanPike]

I am using SQL 2K on one domain and MS Exchange/Windows 2003 server on another domain.
I have set up the Outlook 2003 client on the SQL Server and set up an SQL mail profile.
When in Enterprise manager - Support Services - SQL Mail - Properties I can test the Profile name and it says all is OK.
However when I use the
exec xp_sendmail in Query analyzer I get the following
Server: Msg 18025, Level 16, State 1, Line 0
xp_sendmail: failed with mail error 0x800040005

I think it is to do with seperate domains.
Can anyone give me any clues.

Thanks in advance