When sending outbound email from an email program (like Outlook or Thunderbird) or from a mobile device (like iPhone or Blackberry) that is not using Premium MobileSync, your program or device connects to our outbound email servers using an Internet protocol called “SMTP” (The Simple Mail Transport Protocol).
An email server, however, does lots of different things in addition to sending outbound email. It may allow checking of email via POP or IMAP, or checking your address book using LDAP, or other things. So, when your email program connects to the server it has to specify what it wants to do (i.e. send an email). It does this by connecting to a numbered “port” on the server. Port number “25” is the Internet standard for “regular outbound email”.
However, because port 25 is standard for outbound email, many ISPs, wifi networks, hotels, airports, and other locations that provide Internet access will arbitrarily block any connections to servers (except perhaps their own) on port 25 in order to stop spammers from using their services for the sending of spam, viruses, or malware and to prevent their IP addresses from being black listed.
Read the rest of this post »