PostCast SMTP Server
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Troubleshooting Problems With Anti-Spam Systems

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While I despise spam where irresponsible spammers send millions of e-mails, I do think there is a place for responsible direct e-mail campaigns using legitimate targeted opt-in lists where individuals have given permission to receive information. Various anti-spam tactics are implemented nowadays by ISPs to block spam, but they often harm innocent people and prevent critical business e-mail from being delivered. I will try to explain what methods are commonly used and how you can get around the problems they are causing.

To cut down on the amount of spam e-mail that is sent, some ISPs are blocking port 25 used to send e-mail from your computer. ISPs may block connections from IP addresses that are on their black lists due to excessive amount of spam sent from that addresses and may also perform a relay check to prevent messages that are coming from open SMTP relays. Another popular method to block spam is checking the sender's IP address against list of dynamic IP addresses. If the IP address is dynamic, the e-mail is rejected. 

About backup SMTP servers

The Professional Edition of PostCast Server has a feature that allows you to specify one or more backup SMTP servers. If only certain domains are unable to receive messages from PostCast Server, you can use this option to forward those messages to your ISP's or any third-party SMTP server. If PostCast Server is unable to deliver a message, it can forward it to a backup SMTP server. This enables the program to successfully send higher percentage of messages, but unfortunately does not help if your ISP is blocking port 25. For more information, see the SMTP Gateways topic.

Anti-Spam Methods:

  1. Overview
  2. Port 25 Blocking
  3. Internet Black and White Lists
  4. Blocking Open Relays
  5. DNS Lookups
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