Do you face an issue in which you can see that the mail has delivered to your recipient, but Roundcube still shows “Sending Messages” and just freezes ? Roundcube looks to be freezed !
The issue might be due to X-Frame-Options set to DENY in Apache configurations.
If its an Ubuntu server, check the setting in
/etc/apache2/conf-available/ssl-params.conf
If you find this – Header always set X-Frame-Options DENY
Change it to Header always set X-Frame-Options “sameorigin”
Restart Apache and try Roundcube now.