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Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and if sufficient time has elapsed, the email will be accepted.
Greylisting can refer to: . Greylisting (email), a method of defending e-mail users against spam Greylisting (employment), a form of blacklisting for lesser offenses Hollywood graylist, people who were on the Hollywood blacklist operated by the major studios, but could find work at minor film studios on Poverty Row
Various anti-spam techniques are used to prevent email spam (unsolicited bulk email).. No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) as opposed to not rejecting all spam email (false negatives) – and the associated costs in time, effort, and cost of wrongfully obstructing good mail.
Systems that implement greylisting work fine with VERP if the envelope sender follows the above-mentioned format. However, some VERP implementations use message number or random key as part of VERP, which causes each post to the mailing list to be delayed unless the greylisting system treats "similar" sender addresses as being equivalent.
This may also cause each e-mail to be delayed unless the greylisting system ignores the tag, or whitelists sending hosts that successfully retry. Challenge-response spam filtering and systems that sort mail based on the bounce address (e.g. for removing duplicates) may work less smoothly with BATV-tagged addresses.
Callback verification, also known as callout verification or Sender Address Verification, is a technique used by SMTP software in order to validate e-mail addresses.The most common target of verification is the sender address from the message envelope (the address specified during the SMTP dialogue as "MAIL FROM").
SORBS ("Spam and Open Relay Blocking System") was a list of e-mail servers suspected of sending or relaying spam (a DNS Blackhole List).It had been augmented with complementary lists that include various other classes of hosts, allowing for customized email rejection by its users.
Session Delaying/Greylisting and connection response delaying; Sender validation and recipient validation; Multi-level attachment blocking (based on block lists or allow lists or content based executable blocking) As well as multiple RFC validation mechanisms. Multi-threaded (Since version 2.x) Platform independent (Written in Perl)