Mail subdomain automagically added to the request stack.

Hello,

the Lets Encrypt integration, at some random time, has started to add the "mail"-subdomain to its basic stack, changing the normal domain stack of "domain.tld"+"www.domain.tld", and adding an unexisting subdomain, which I don't even want to use for most virtual servers I run. This is resulting in retries per 5 minutes, after failing and thus spamming my mailboxes. This behaviour has been active for a while now.

What's the actual reason this behaviour got deployed? I can't see any reason why this subdomain should be in the main domain stack?

So... what's the deal? :)

TYIA!