Knowledgebase: Web hosting FAQ
Open Relays in RBLs :: SPAM databases
Posted by - NA - on 19 September 2005 01:40 AM
Open Relays in RBLs (ie. SPAM databases)

Open relays are e-mail servers that are configured to accept and transfer e-mail on behalf of any user anywhere, including unrelated third parties. If your computer acts as an open relay, it allows any e-mail

How spammers detect open relays

Spammers use automated software to scan the Internet trying to find open relays. If they find out that your server is open, they will probably send spam through it. The software they are using scans a range of Internet IP addresses by trying to establish a network connection on port 25. If the connection succeeds, an IP address is listed and used for sending.

There are at least two advantages for the spammers:

1. This technique lets spammers hide their identities because it appears that the spam actually comes from you. This makes extremely hard to track them down.

2. It is virtually impossible to get caught by their ISP. All ISPs deny sending spam from their networks. If the spammers cannot be tracked down, they cannot be reported to their ISP which would broke down their account anyway, because of violating the Acceptable User Policy.

Recipients of the spam sent from your computer could flood your server with complaints. The spam and resulting e-mail traffic could overwhelm your system. If you are maintaining an open relay, you are leaving your door open to the theft of your computer services.

How ISP/Webservers reject messages from open relays

When you send messages from an SMTP server running on your computer, some ISPs perform a relay check. They identify your computer's IP address and try to establish a connection to port number 25 which is the port used to send e-mail. If the server on your computer accepts the connection - your message is rejected.

For RBLs (i.e. SPAM databases), kindly follow the below mentioned URL:

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