Certbot and Let's Encrypt Now Support IP Address Certificates
eff.org
eff.org
Related 6-Day and IP Address Certificates Are Generally Available (506 points, 2 months ago, 281 comments) https://news.ycombinator.com/item?id=46647491
Just the IP will most of the time be dynamic, and you might have your IP changing regularly.
If you use HTTP-01 challenge method you require an HTTP server on the host.
If you don't want an HTTP server on your imap/smtp server you need to use the DNS-01 challenge method.
Meant to say those solutions are in addition to Lets Encrypt. An X509 certificate is an X509 certificate, regardless if its for https, imaps, or smtps. If you're distributing your stuff across multiple hosts or containers, then it makes sense to use some sort of automation, configuration management, or certificate management/distribution system.