Open relay
An open relay is a mail server configured to accept and forward email from any sender to any recipient, without verifying that the sender is an authorized user of that server or domain. Mail servers are meant to relay only for their own authenticated users or a defined list of trusted sources; a misconfigured server that relays for anyone becomes a free, anonymous sending point that spammers actively scan for and abuse, often without the operator noticing until deliverability problems appear. Once abused, an open relay's IP typically ends up listed on DNS-based blocklists (DNSBLs), which then causes legitimate mail from that same server to be rejected or quarantined by other mail systems. Testing for the specific cause behind a blacklist listing, including whether a mail server is acting as an open relay, is the first step before requesting delisting from the list that flagged it.
By AdminKit team, Netbase JSC · Updated