Why can't you just change the VirtualHost for the webmail subdomain so that it does not redirect HTTP to HTTPS?
There are many ways to manage redirects for this. Here is one example
Update:
Oh, you should avoid using -f (force). This is needed only for rare specific cases. It does not force issuance or bypass any problems. And, when used incorrectly, like here, it can lead you to become Rate Limited.