This only applys to clients who use a 3rd party email client like (outlook 2007-2010, Thunderbird) or others ones that are out there.
Some Internet Service Providers block port 25 which is the standard email protocol port.
If you are able to receive email but you are not able to send it we suggest trying port 26 as
we run a second copy of our mail servers for those who are unable to use port 25.
