Milter (portmanteau for mail filter) is an extension to the widely used open source mail transfer agents (MTA) Sendmail and Postfix. It allows administrators to add mail filters for filtering spam or viruses in the mail-processing chain. In the language of the art, "milter" refers to the protocol and API implementing the service, while "a milter" has come to refer to a filter application that uses milter to provide service.
Prior to the advent of milter, an email filter was generally implemented as a program to which an MTA would hand the message once it has completely arrived, with most of the message's envelope information removed. That program could then analyze the header and body of the message and make a decision to accept the message (i.e. return a "success" status to the MTA) or reject it (i.e. return a "failed" status to the MTA). The MTA would then log a successful delivery or return a failure message to the sender as appropriate, and the filter would be responsible for affecting the delivery of the message (to the intended inbox(es) as-is, or modified to remove unwanted content, or to specific folder(s), etc.).
An MTA that is milter-capable instead notifies filters to which it is connected about each phase of the delivery of a message, from initial client connection through completion of transmission. At each phase of the SMTP session, the filter is given data about the arriving message and then has an opportunity to terminate acceptance of the message early when appropriate. For very large messages, this can have an enormous impact when a decision to reject can be made as early as possible. Moreover, unlike the former model, a milter-capable MTA can connect to multiple filters in parallel that serve specific purposes such as anti-virus, anti-spam, message authentication, flow regulation, etc. Finally, such filters can take special action on the message: add or remove recipients in the envelope; alter the body prior to delivery; add, change or remove header fields in the message, etc.
The Sendmail Consortium includes a special thread-based library in the sendmail distribution that provides the milter API.
MTAsedit
Sendmailedit
Milter was first officially introduced to Sendmail 8.12.0 released on September 8, 2001.[1] Milter was available in sendmail 8.11.x versions as an FFR (For Future Release/Beta code). Milter has undergone substantial improvements since its first official release.
Postfixedit
Milter support was added to Postfix 2.3.0 released July 12, 2006.[2][3] Many, but not all, milters can be used with Postfix "out of the box".
Interfaceedit
The Sendmail Consortium provides a documented Application Programming Interface that can be used to create milters for special needs. The library provided by The Sendmail Consortium is pthread-based. However, documentation of the protocol used for communication between sendmail and milter processes is not provided. This internal protocol is subject to changes in new sendmail versions.
milter, other, uses, disambiguation, portmanteau, mail, filter, extension, widely, used, open, source, mail, transfer, agents, sendmail, postfix, allows, administrators, mail, filters, filtering, spam, viruses, mail, processing, chain, language, milter, refers. For other uses see Milter disambiguation Milter portmanteau for mail filter is an extension to the widely used open source mail transfer agents MTA Sendmail and Postfix It allows administrators to add mail filters for filtering spam or viruses in the mail processing chain In the language of the art milter refers to the protocol and API implementing the service while a milter has come to refer to a filter application that uses milter to provide service MilterTypeMTA extensionWebsitemilter org at the Wayback Machine archived 2015 05 10 Contents 1 History 2 MTAs 2 1 Sendmail 2 2 Postfix 3 Interface 4 Sample Milters 5 ReferencesHistory editPrior to the advent of milter an email filter was generally implemented as a program to which an MTA would hand the message once it has completely arrived with most of the message s envelope information removed That program could then analyze the header and body of the message and make a decision to accept the message i e return a success status to the MTA or reject it i e return a failed status to the MTA The MTA would then log a successful delivery or return a failure message to the sender as appropriate and the filter would be responsible for affecting the delivery of the message to the intended inbox es as is or modified to remove unwanted content or to specific folder s etc An MTA that is milter capable instead notifies filters to which it is connected about each phase of the delivery of a message from initial client connection through completion of transmission At each phase of the SMTP session the filter is given data about the arriving message and then has an opportunity to terminate acceptance of the message early when appropriate For very large messages this can have an enormous impact when a decision to reject can be made as early as possible Moreover unlike the former model a milter capable MTA can connect to multiple filters in parallel that serve specific purposes such as anti virus anti spam message authentication flow regulation etc Finally such filters can take special action on the message add or remove recipients in the envelope alter the body prior to delivery add change or remove header fields in the message etc The Sendmail Consortium includes a special thread based library in the sendmail distribution that provides the milter API MTAs editSendmail edit Milter was first officially introduced to Sendmail 8 12 0 released on September 8 2001 1 Milter was available in sendmail 8 11 x versions as an FFR For Future Release Beta code Milter has undergone substantial improvements since its first official release Postfix edit Milter support was added to Postfix 2 3 0 released July 12 2006 2 3 Many but not all milters can be used with Postfix out of the box Interface editThe Sendmail Consortium provides a documented Application Programming Interface that can be used to create milters for special needs The library provided by The Sendmail Consortium is pthread based However documentation of the protocol used for communication between sendmail and milter processes is not provided This internal protocol is subject to changes in new sendmail versions Sample Milters editMIMEDefang AmavisReferences edit SENDMAIL RELEASE NOTES 8 12 0 8 12 0 Sendmail 2001 09 08 Retrieved 2020 08 03 Postfix before queue Milter support Limitations Postfix Retrieved 2020 08 03 Wietse Venema Postfix 2 3 Change log 20060301 20060515 Retrieved 2020 08 03 Retrieved from https en wikipedia org w index php title Milter amp oldid 1161430225, wikipedia, wiki, book, books, library,
