Anti-spam Efforts
Many Internet service providers use DNSBLs (DNS-based Blocking Lists) to disallow mail from open relays. Once a mail server is detected or reported that allows third parties to send mail through them, they will be added to one or more such lists, and other e-mail servers using those lists will reject any mail coming from those sites. The relay need not actually be used for sending spam to be blacklisted: instead, it may be blacklisted after simple test that just confirms open access.
This trend reduced the percentage of mail senders that were open relays from over 90% down to well under 1% over several years. This led to spammers adopting other techniques, such as the use of botnets of zombie computers to send spam.
One consequence of the new unacceptability of open relays was an inconvenience for some end users and certain internet service providers. To allow customers to use their e-mail addresses at Internet locations other than the company's systems (such as at school or work), many mail sites explicitly allowed open relaying so that customers could send e-mail via the ISP from any location. Once open relay became unacceptable due to abuse (and unusable due to blocking of open relays) ISPs and other sites had to adopt new protocols to allow remote users to send mail. These include smart hosts, SMTP-AUTH, POP before SMTP, and the use of virtual private networks (VPNs). The IETF has written a best current practices covering Email Submission Operations in RFC 5068.
Note that the above only becomes an issue if the user wishes to (or has to) continue to send e-mail remotely, using the same SMTP server which they were previously accessing locally. If they have valid access to some other SMTP server from their new, remote location, then they will typically be able to use that new server to send e-mails as if from their old address, even when this server is properly secured. (Although this may involve some reconfiguration of the user's E-mail client which may not be entirely straightforward.)
The Can Spam Act of 2003 makes it illegal to send spam through an open relay in the United States, but makes no provision regarding sending personal e-mail through them or regarding their operation, although the effectiveness of the act has been questioned.
Read more about this topic: Open Mail Relay
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