Some ISPs block email on the standard SMTP port of 25. There are several ways around this if you find you cannot send email from one particular location but can in others (for example if you can email from your laptop at home, but not on a public hotel Wifi).
The easiest way to bypass this problem is to enable SSL in your email client and change the port to 465. Firstly this encrypts all traffic between you and the mail server so the ISP won't be able to see what the traffic is and secondly it will bypass any outright blocks they have put on the default port.
If you have tried the above method of using SSL and that hasn't helped, you can request that we add additional SMTP ports by logging a ticket with the Dedicated Hosting Support team from your Namesco Online Control Panel stating that you would like additional SMTP ports opened on your VPS/Server.