I’ve always found clusters a pain in the neck.  For my latest pain I just spent the last 2 days debugging DBMail on an instance on a cluster.  To the best of my ability everything appeared set up correctly but when I tried a test email it wouldn’t send.  The log showed “Successful Activation” but in sysmail_allitems all I saw was “unsent” for the emails.  To shorten a long story if you have a clustered environment and your instance isn’t using port 1433 you have to create a local TCPIP alias for the instance using the correct IP address.

 What a pain!