are they in the same forest?
or is it a trust setup?
does the FQDN resolve?

have you tried switching it off and on again?

the From field is the trick but it has to be from another exchange object, it cant be just an alias on the same mailbox.
This is because outlook & exchange resolve address's down to the SID not the address itself.
The common method is to create a 2nd mailbox that forwards incoming mail to the primary mailbox but allows you to select a different 'from' object when sending. (need to setup the send as privledges)
a similar setup can be achieved using a distribution group with send as rights.

Outlook 2010 improves the dual mailbox setup as you can add two exchange accounts to a single outlook profile.

Thanks phase, I had no idea you could add 2 exchange accounts under Outlook 2010

Thanks phase, I had no idea you could add 2 exchange accounts under Outlook 2010

This is probably a single most requested feature in outlook since day dot. I can't believe it's taken MS this long to implement it :/ In any case Outlook 2010 is awesome, I like it even more than 2007.

Also what Phase said. :)

Yeah even the iPhone beat Outlook to multiple exchange accounts, how embarrassing

I didn't want separate profiles you dildo