If you get the below error while sending emails from rouncube.

SMTP Error: SMTP error: Connection failed: Failed to connect socket: Connection timed out in /usr/local/cpanel/base/3rdparty/roundcube/program/steps/mail/func.inc on line 1418

It happens if you have CSF firewall installed and it is upgraded automatically or something like that. Just make following changes in to firewall configuration and it will fix the issue

root@server[#] vi /etc/csf/csf.conf
and search for
SMTP_ALLOWLOCAL = 1
Replace it with
SMTP_ALLOWLOCAL = 0

Save and Quit.

That’s all your are done.

Note : Restart the csf service after making the  changes.