Figured it out. I needed the --smtp-tls option, and I needed to drop the --smtp-authtype option, which has been deprecated by svnnotify.
It would be nice if someone could update the '--help' text to reflect these changes. Currently, the help text gives --smtp-authtype as a valid option (no mention of it being deprecated) and --smtp-tls is not mentioned in the help text at all.
Thanks,
David