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We’ve heard a lot about whitelisting and blacklisting (actually more accurately and appropriately called ‘blocklisting’, not blacklisting), but many are unfamiliar with the term “greylisting” and, to a lesser extent, “rate limiting.” Greylisting and rate limiting are two additional but lesser used methods which some ISPs and other email receivers may employ to attempt to deal with spam and/or a sudden influx of bulk email. Greylisting is when the receiving mail server temporarily rejects the email and bounces it back to the sending email server with a “try again later” message; if the sender’s information is legitimate their server will receive the bounce and try to send the email again, if it isn’t legitimate it won’t receive the bounce message at all. Rate limiting is when the receiving ISP detects a sudden unexplained or unpermitted surge of email (such as occurs with a spam run) and responds by throttling the inbound mail stream. Rate limiting can result in email being delayed, or even discarded.
As with blocklisting prevention, it is important that you provide a means of opt-in for all email marketing materials, and ideally a double opt-in option. Also remember that email list maintenance is vital, so be sure you are routinely checking your email list and ensuring that it is up to date.