When trying to send e-mail following error appears like:
Forbidden You don't have permission to access /imp/compose.php on this server
Error when communicating with the server
There can be also similar error in apache error log file:
[error] [client 18.104.22.168] ModSecurity: Access denied with code 403 (phase 2). Match of "eq 0" against "MULTIPART_UNMATCHED_BOUNDARY" required. [file "/etc/httpd/conf.d/mod_security.conf"] [line "70"] [msg "Multipart parser detected a possible unmatched boundary."] [hostname "HOSTNAME"] [uri "/horde/imp/compose.php"] [unique_id "8m0u-n8AAAEAAD7blhoAAAAO"]
ModSecurity rules prevent Horde/Roundcube webmail work.
Configure mod_security in a way that will allow incoming and outgoing connections to utilise ports that are used by Webmail software. Plesk for Linux Webmail is available for access through port 80 via webmail.domain.com.
Alternatively, you could disable mod_security in Plesk GUI or its separate rules:
Plesk > Tools & Settings > Web Application Firewall (ModSecurity).