Delay Tactics
Some programs are designed to insert delays in mail delivery to filter out non-RFC compliant mail servers. Spammers have traditionally used zombie PCs and custom mail servers designed for speed over compliance. By inserting delays or outright rejecting initial attempts to deliver mail, these non-compliant servers can be identified and any mail sent by them can be dropped.
Open-Source Solutions
- GreetDelay
- GreetDelay is a very simplistic program designed to insert a user-defined delay between initial remote connections and the actual SMTP greeting. In addition, the program can be run in strict mode which will cause an error message and the connection will be dropped if the remote server sends any commands prior to the greeting.
- SMTP Delay
- SMTP Delay is a slightly more advanced version of the GreetDelay program. In addition to adding the delay, the reverse DNS for the connecting IP is checked. The delay time is varied based on whether the reverse DNS is valid, and whether the reverse DNS appears to be for a dynamic IP or not.
- qMail Banner
- qMail Banner is intended to deliver a banner message to connecting mail servers. It offers the ability to insert delays both before and after the banner is displayed. If the remote server sends commands or closes the connection before the banner and delays have been completed, the program can set environment variables which are passed on to the next program run.