I just perform a Exch2010 installation with coexistence with Exch2003.
Everything seem to be fine at the moment.
In this org. have several user still using OE 6.
If use OE6 send email to external domain result in “The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ‘[email protected]’. Subject ‘a’, Account: ‘Exadmin’, Server: ‘192.168.100.3’, Protocol: SMTP, Server Response: ‘550 5.7.1 Unable to relay’, Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79”
My Outgoing Server requires authentication is tick–> Use same as my incoming.
For troubleshooting, one the same PC, if using MS Outlook able to send the same email to external without error.