recipient can't decrypt my message

I have tested with Adele and one live recipient and my setup works. A recipient uses Startmail with GnuPG and my messages come in to him garbled but don’t appear to his client as being encrypted, other than the cyphertext. His encrypted correspondence with others works well.
I have his public key and he has mine.

Someone from an Enigma forum suggested I opt for PGP/MIME in the Enigmail Preferences. Bingo, that did the trick.