Many people who travel with a laptop computer encounter difficulties sending email when they are at a remote location. Or you may have switched ISPs and now you can't send email. See the diagram at right.

The scenario is typically that the user has an email client program such as Microsoft Outlook or Mozilla Thunderbird that is configured to send and receive email from a known email server.

Everything works fine from the user's normal place of work such as their company office or home office. But when working from a remote location, the user is often able to receive their email OK, but might encounter difficulties or delays when sending email they have just written and wish to send.

Although there are several possible reasons why this may occur, the most common seems to be blocking of SMTP port 25 by the remote Internet Service Provider (ISP). Port 25 is the TCP port used to send email to your email server. This blocking is done to prevent users from sending out Spam using the anonymity of the Internet access provided by the remote ISP, and the ISP wants to control unwanted activity.

In many cases the remote ISP intercepts your port 25 requests and sends your email to the recipient through their email servers. For example, we have run tests which show that this is done by the Internet service known as T-Mobile Hotspot available in Starbucks and other locations. In other cases your port 25 traffic may be blocked entirely. We have noticed this with several ISPs, who seem to prefer that you only use their SMTP servers to send your email.

Most of the time this "email interception" is not a problem but in some cases you may not want sensitive email to be delivered without your control, or your email may need to be archived on your company's server for compliance or retention reasons. In other cases your email may be very slow to send, or perhaps you might be completely unable to send. To fix this issue for our clients, we have opened additional ports on True Blade's email servers which are not commonly blocked. If you have access to webmail on your company's server, that is another option. Talk to your system administrators or True Blade for additional suggestions to deal with port 25 blocking from remote ISPs.