In case it's news to others, as it was to me, Fastmail routinely acquires SSL certificates for its customers' domains without their knowledge or consent.
In case it's news to others, as it was to me, Fastmail routinely acquires SSL certificates for its customers' domains without their knowledge or consent.
We still need to find a way to provide automatic SSL for customer domains though - because we allow our customers to create arbitrary websites inside either their domains or their personal subdomain on our domains (username.fastmail.com).
The alternative of NOT doing something with SSL certificates is having insecure websites for customers by default, which will be more and more punished (and rightfully so) by browser interfaces. Setting up SSL for the domains which are hosted with us is the right thing to do.
The short-term nature of Let's Encrypt also works out well for this, because if you take your domain elsewhere, FastMail loses the certificate to claim to be that domain very quickly due to rapid expiration.