In short, filtering happens when your ISP blocks port 25, the SMTP mail port, to any outside systems. To check this, follow these steps:
(Please make sure you can browse the Internet, and thus have a live established Internet connection, before performing this test.)
(Without the quotes, and where mail.domain.com is your Outgoing SMTP Mail Server name you are trying to test).
If it works, you will get something SIMILAR the following:
Connected to satexas.com.
Escape character is '^]'.
220 rooster.satexas.com ESMTP Sendmail 8.15.11/8.15.11; Thu, 24 Feb 2005 00:45:48 -0600 (CST)
If you get absolutely no response, you are being blocked. To confirm this, we suggest you call your ISP's technical support team. If you have a "static IP address", they will often remove this blocking.
WARNING: When calling into your ISP, keep it simple and just ask them about 'Port 25 SMTP Blocking' on your mail. They do not need to know your Domain name, or any other details about your mail account because they DO NOT handle your mail. Giving them too much information will likely call the "Level 1 Support Tech" you are talking to, to give you false information and thus make your problem only more confusing. Simply give them the name of the mail server you are trying to use (IE mail.domain.com), and ask them if they are blocking you sending mail to that domain on Port 25.