w | mailx -s "test to one email" email@example.com
And it works fine.
Then I created an Exchange distribtution group, with me as the sole member, and tested:
w | mailx -s "test to group" DST-Pinnacle_Backups-ROV@mywork.com.au
Didn't work. Why?
The trick turns out to be that Exchange distribution groups have, by default, a setting that requires senders to "authenticated" - this seems to mean "part of my Windows domain" - in that they have to be authenticated as Windows users. Clearly, sendmail on Solaris isn't going to to meet this criterion.
The solution: open properties for the distribution group, and click the "MailFlow Settings":
Then double-click "Message Delivery Restrictions":
Un-select "Require that all senders are authenticated". Simples!