Many ISP SMTP servers are starting to require authentication or SSL, so it isn't that out of scope. Digitally signing or encrypting messages would be a stretch in my opinion.
There were a decent set of changes to log4j's SMTPAppender in the last year or so to improve configurability and authentication. You should check that code for any new properties to try to maintain consistency.
I'm think that SSL encrypting of the traffic between log4j and a SMTP server is handled by JavaMail and is transparent to log4j, but I could be wrong.
Use of certificates to digitally sign log emails seems wrong since it would be affixing someone's identity to a email message without their involvement. But maybe you were talking about the SSL certificates instead.