Situation ispconfig v3 server myserver.mydomain.ext with an ssl (via LE4ISPC) mail.anotherdomain.ext mail.yetanotherdomain.ext in the past it was working but now gmail has some extra checks. Is it possible to get an ssl on the extra mail..... ?
You could add those other names as vhost aliases of the myserver.mydomain.ext site, up to 100 names in a certificate. Then pay attention that the customers don't change DNS for their mail subdomain name, or cancel/move service without telling you or your server's certificate won't renew anymore.
