For those who didn't know, Let's Encrypt is a free, automated and open Certificate Authority. In short terms, you can now obtain a free certificate trusted by everyone.
Modoboa's official installer has been improved to include Let's Encrypt as a certificate provider during the installation process. To activate this feature, edit the installer.cfg file and apply the following modifications:
[certificate]
generate = true
type = letsencrypt
[letsencrypt]
email = admin@example.com
Change email to a valid value since it will be used for account recovery.
Please note this option requires the hostname you're using to be valid (ie. it can be resolved with a DNS query) and to match the server you're installing Modoboa on.
You really have no excuse not to install your own mail server ;-)