@bloops are you saying you created a Domain called mail.domain.tld ? This is not needed. I think the certificate issue could simply be a matter of DNS propagation. Especially, with manual DNS, depending on your provider, this can take a while and Cloudron has no way to check . In such cases, it will start using a self signed certificate initially. There is a nightly cron job to renew certs and that would kick in and get a certificate eventually.