Next big update will be support for subdomains and for other domains (HestiaCP already has support for it).
What you can do now is to (as admin)
- go to WEB -> Edit (edit yourhostname)
- add mail.somedomain.com as Alias
- Save it
- On the same page turn off LetsEncrypt, and Save it
- On the same page turn on LetsEncrypt, and Save it