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!