I've posted a bug report on CSF lately, but it seems that it's something cPanel related actually:
Why do I think that?
Because on one of our servers:
Centos 5 x86_64 (on a Core 2 Duo)
I CAN send emails via Roundcube, but on other webserver :
Centos 4 i386 (on a Pentium D)
I CANNOT send emails via Roundcube.
Has there been made any fix or changes since this 2 versions on the Roundcube smtp method ?
CSF SMTP_BLOCK does block all outgoing traffic from the server to anywhere that's not from root / mail / mailman; although all webmails used to work because they connected to localhost, and localhost was 100% allowed to be used by anyonw (ALLOW_LOCAL on the firewall).
Maybe if you changed the SMTP auth from roundcube or some sorta setup change so that roundcube doesn't send emails as mail or root and it doesn't uses smtp sock to localhost that may be what's failing.
PS: CSF + SMTP_BLOCK + ALLOW_LOCAL is installed on both servers, and I meant it to be this way to avoid spammers.
PS2: Squirremail and Horde works correctly on both.