On Shared server as well  as on Dedicated server some time we are facing large connection issue to SMTP port 25 at that time mostly we disable SMTP port 25 and enable any other port for SMTP but after changing SMTP port mostly we receive following error message in Horde webmail.

There was an error sending your message: Failed to connect to localhost:25 [SMTP: Invalid response code received from server (code: 421, response: Too many concurrent SMTP connections; please try again later.)]

To resolve above error simply change SMTP port from 25 to new SMTP port  in following file.

root@server [/usr/local/cpanel/base/horde/imp/config]# Pico servers.php

And change following line

From

‘smtpport’ => 25,

To

‘smtpport’ => 26,

We have taken new port as 26 for example you can use any port as per your requirement.

Similar problem with “Squirrelmail” then refer following steps.

root@server [/usr/local/cpanel/base/3rdparty/squirrelmail/config]#pico config_default.php

Change following line

From

$smtpPort = 25;

To

$smtpPort = 26;

Save file and exit and now open the webmail.