When will they let me generate certificates for IMAP and SMTP?
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.