Postfix Tweaks for Sending from Remote Email Clients

Due to protections against spam, sending email from remote clients often requires special tweaks. This post shows how to configure postfix (a very popular mail transfer agent for Linux) to allow for sending mail with remote clients.
Many ISPs block port 25, the default smtp port, in order to make it harder for spam bots to operate. You can enable an alternate port by editing the master.cf file, usually found in /etc/postfix (it's location will depend on how it was installed).
In master.cf find a line similar to the following.

smtp inet n - n - - smtpd -o content_filter=spamassassin

After this line, add an identical line, but change stmp at the beginning to and unused port number, e.g., 2525.

It should now look something like this:

smtp inet n - n - - smtpd -o content_filter=spamassassin
2525 inet n - n - - smtpd -o content_filter=spamassassin

Now restart postfix:

/sbin/service postfix restart

You can test your connection from a remote computer with the command:

telnet your_domain_name.com 2525

If it appears you have a connection, type:

EHLO your_domain_name.com

Authentication is working if you get a response similar to the following.

250-AUTH PLAIN LOGIN

The above was tested on at Godaddy Virtual Dedicated Server (VDS) running Centos 5, and sending mail from and HTC Android phone.

Reference: http://rimuhosting.com/support/settingupemail.jsp?mta=postfix&t=smtp#smtp