RewriteEngine on
RewriteLogLevel 9
RewriteLog "/var/log/apache2/rewrite.log"
RewriteCond %{HTTP_HOST} mail.domain.com [NC]
RewriteRule . https://www.domain.com/mail/  [L]
RewriteCond %{REQUEST_URI} mail.domain.com [NC]
RewriteRule . https://www.domain.com/mail/  [L]
RewriteCond %{REMOTE_HOST} mail.domain.com [NC]
RewriteRule . https://www.domain.com/mail/  [L]