All email sent via the Internet is routed through a protocol called SMTP (Simple Mail Transfer Protocol) that uses by default TCP port 25. Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to send through email servers other than those belonging to their ISP.

How to determine if TCP port 25 is blocked on your network.

From the console (command prompt) type the following

telnet smtp.myisp.com 25

Results for port 25 not blocked

If your ISP hasn't blocked port 25 then the returning result will look as follows:

Trying x.x.x.x

Connected to smtp.myisp.com.

Escape character is '^]'.

220 smtp.myisp.com ESMTP Tue, 18 Apr 2011 15:43:00

Results for port 25 blocked

If your ISP has blocked port 25 then the returning result will look as follows:

Trying x.x.x.x

telnet: connect to address x.x.x.x: Connection refused

telnet: Unable to connect to remote host

What can I do if port 25 is blocked?

If your ISP has blocked port 25 then the simplest solution is to use a mail relay service (as known as SmartHost). outMail is a outbound SMTP service which supports port 25 and alternative ports such as tcp port 587. The outMail service is safe and secure service using SMTPAuth for username and password authentication. outMail can be used on any device supporting the SMTP protocol from any location.

About Prolateral

Prolateral Consulting is in business to put your organisation back in control of your own Information Technology, specialising in information and messaging security, computer forensic services, and disaster recovery planing.

Contact Info

Prolateral Consulting Ltd
Luton, Bedfordshire, UK
Tel : +44 (0) 8450 763760
Email : This email address is being protected from spambots. You need JavaScript enabled to view it.

Instant Information request

Please complete the request for information if you wish to discuss matters further or if your needs are more urgent then you can request a call back from us.