From RootdevWiki
The Simple Way
sudo /usr/sbin/sendmail -q -v
Flush the clientmqueue
sendmail -Ac -qf -v
Debugging
If you are getting "host map: lookup (domain): deferred" errors:
It's most likely a mailertable or DNS lookup issue. Check your mailertable is valid, or use the d8 debugging syntax as follows:
sendmail -qf -v -d8.32
