Configure Squirrelmail

Change domain in /etc/squirrelmail/config.php:

$domain = 'mydomain.com';

so the email received will have user@mydomain.com

To use https on apache, add the following to /etc/httpd/conf.d/squirrelmail.conf:

<Location /webmail>RewriteEngine on
RewriteCond %{HTTPS} !=on
RewriteRule .* https://%{HTTP_HOST}:443%{REQUEST_URI} [QSA,R=permanent,L]
</Location>

Finally, selinux settings:

setsebool -P httpd_can_network_connect=1