Below is the commom error with the Horde which can be Fix using the steps :
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
‘smtpport’ => 25,
‘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
$smtpPort = 25;
$smtpPort = 26;
Save file and exit and now open the webmail.