Open Relays in RBLs i.e. SPAM databasesOpen relays are email servers that are configured to accept and transfer email on behalf of any user anywhere, including unrelated third parties. If your computer acts as an open relay, it allows any email.
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 use, 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
- 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.
- 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 break down their account anyway, because of violating the Acceptable User Policy.
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 email. If the server on your computer accepts the connection - your message is rejected.
Note: It is recommended to use email list validation service before starting an email campaign.