Welcome back @smahi
Check this tutorial
and follow it accordingly. There must be another rewrite rule that conflicting with this one.
RewriteEngine on
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^$ https://serverdomain.com/snappymail? [R=301,L,NC]
RewriteRule ^$ https://serverdomain.com/snappymail? [R=301,L,NC]
: This line defines the redirection rule. If the query string is empty, the request will be redirected to the snappymail
page on the serverdomain.com
domain with a 301 Moved Permanently status, a Location header, and no cache.