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o a non 8bitmime peer results in a bounce this is allowed by rfc 6152 40 smtp auth edit main article smtp authentication the smtp auth extension provides an access control mechanism it consists of an authentication step through which the client effectively logs into the mail server during the process of sending mail servers that support smtp auth can usually be configured to require clients to use this extension ensuring the true identity of the sender is known the smtp auth extension is defined in rfc 4954 smtp auth can be used to allow legitimate users to relay mail while denying relay service to unauthorized users such as spammers it does not necessarily guarantee the authenticity of either the smtp envelope sender or the rfc 2822 from header for example spoofing in which one sender masquerades as someone else is still possible with smtp auth unless the server is configured to limit message from addresses to addresses this authed user is authorized for the smtp auth extension also allows one mail server to indicate to another that the sender has been authenticated when relaying mail in general this requires the recipient server to trust the sending server meaning that this aspect of smtp auth is rarely used on the internet 41 smtputf8 edit supporting servers include postfix version 3 0 and later 42 exim experimental as of the 4 86 release quite mature in 4 96 sendmail experimental support in 8 17 1 dovecot imap pop3 experimental support in 2 4 0 momentum versions 4 1 43 and 3 6 5 and later communigate pro as of version 6 2 2 44 courier mta as of version 1 0 45 halon as of version 4 0 46 microsoft exchange server as of protocol revision 14 0 47 haraka and other servers 48 oracle communications messaging server as of release 8 0 2 49 security extensions edit mail delivery can occur both over plain text and encrypted connections however the communicating parties might not know in advance of other party s ability to use secure channel starttls or opportunistic tls edit main articles opportunistic tls and email encryption the starttls extensions enables supporting smtp servers to notify connecting clients that it supports tls encrypted communication and offers the opportunity for clients to upgrade their connection by sending the starttls command servers supporting the extension do not inherently gain any security benefits from its implementation on its own as upgrading to a tls encrypted session is dependent on the connecting client deciding to exercise this option hence the term opportunistic tls starttls is effective only against passive observation attacks since the starttls negotiation happens in plain text and an active attacker can trivially remove starttls commands this type of man in the middle attack is sometimes referred to as striptls where the encryption negotiation information sent from one end never reaches the other in this scenario both parties take the invalid or unexpected responses as indication that the other does not properly support starttls defaulting to traditional plain text mail transfer 50 note that starttls is also defined for imap and pop3 in other rfcs but these protocols serve different purposes smtp is used for communication between message transfer agents while imap and pop3 are for end clients and message transfer agents in 2014 the electronic frontier foundation began starttls everywhere project that similarly to https everywhere list allowed relying parties to discover others supporting secure communication without prior communication the project stopped accepting submissions on 29 april 2021 and eff recommended switching to dane and mta sts for discovering information on peers tls support 51 rfc 8314 officially declared plain text obsolete and recommend always using tls for mail submission and access adding ports with implicit tls dane for smtp edit rfc 7672 introduced the ability for dns records to declare the encryption capabilities of a mail server utilising dnssec mail server operators are able to publish a hash of their tls certificate thereby mitigating the possibility of unencrypted communications 52 microsoft expects to enable full smtp dane support for exchange online customers by the end of 2024 53 smtp mta strict transport security edit a newer 2018 rfc 8461 called smtp mta strict transport security mta sts aims to address the problem of active adversaries by defining a protocol for mail servers to declare their ability to use secure channels in specific files on the server and specific dns txt records the relying party would regularly check existence of such record and cache it for the amount of time specified in the record and never communicate over insecure channels until record expires 50 note that mta sts records apply only to smtp traffic between mail servers while communications between a user s client and the mail server are protected by transport layer security with smtp msa imap pop3 or https in combination with an organizational or technical policy essentially mta sts is a means to extend such a policy to third parties in april 2019 google mail announced support for mta sts 54 smtp tls reporting edit protocols designed to securely deliver messages can fail due to misconfigurations or deliberate active interference leading to undelivered messages or delivery over unencrypted or unauthenticated channels rfc 8460 smtp tls reporting describes a reporting mechanism and format for sharing statistics and specific information about potential failures with recipient domains recipient domains can then use this information to both detect potential attacks and diagnose unintentional misconfigurations in april 2019 gmail announced support for smtp tls reporting 54 spoofing and spamming edit main articles anti spam techniques and email authentication the original design of smtp had no facility to authenticate senders or check that servers were authorized to send on their behalf with the result that email spoofing is possible and commonly used in email spam and phishing occasional proposals are made to modify smtp extensively or replace it completely one example of this is internet mail 2000 but neither it nor any other has made much headway in the face of the network effect of the huge installed base of classic smtp instead mail servers now use a range of techniques such as stricter enforcement of standards such as rfc 5322 55 56 domainkeys identified mail sender policy framework and dmarc dnsbls and greylisting to reject or quarantine suspicious emails 57 implementations edit main articles list of mail server software and comparison of mail servers see also edit bounce address cram md5 a sasl mechanism for esmtpa rfc 2195 email email encryption dkim ident list of mail server software list of smtp server return codes pop before smtp smtp after pop internet message access protocol binary content extension rfc 3516 sender policy framework spf simple authentication and security layer sasl rfc 4422 smtp authentication variable envelope return path comparison of email clients for information about smtp support notes edit the history of electronic mail archived december 2 2017 at the wayback machine tom van vleck it is not clear this protocol was ever implemented the first network email ray tomlinson bbn picture of the first email computer by dan murphy a pdp 10 dan murphy s tenex and tops 20 papers archived november 18 2007 at the wayback machine rfc 524 a proposed mail protocol crocker david h december 1977 framework and functions of the ms personal message system pdf the rand corporation archived pdf from the original on may 13 2022 retrieved april 17 2022 rfc 469 network mail meeting summary rfc 733 21 november 1977 standard for the format of arpa network text message a history of e mail collaboration innovation and the birth of a system washington post may 20 2023 issn 0190 8286 retrieved july 7 2024 mckenzie alexander 2011 inwg and the conception of the internet an eyewitness account ieee annals of the history of computing 33 1 66 71 bibcode 2011iahc 33a 66m doi 10 1109 mahc 2011 9 issn 1934 1547 s2cid 206443072 barber d and j laws a basic mail scheme for ein inwg 192 february 1979 ien 85 ietf ien 113 ietf internet experiment note index www rfc editor org retrieved july 7 2024 1 2 postel j august 1982 simple mail transfer protocol report rfc editor doi 10 17487 rfc0821 tldp org archived from the original on august 17 2007 retrieved august 25 2007 barber stan o december 19 2000 draft barber uucp project conclusion 05 the conclusion of the uucp mapping project archived from the original on october 13 2007 retrieved august 25 2007 the article about sender rewriting contains technical background info about the early smtp history and source routing before rfc 1123 eric allman 1983 sendmail an internetwork mail router pdf bsd unix documentation set berkeley university of california archived pdf from the original on may 20 2013 retrieved june 29 2012 craig 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jiankang yao december 19 2014 chinese email address eai mailing list ietf archived from the original on october 2 2015 retrieved may 24 2016 smtp service extension parameters iana archived from the original on may 28 2019 retrieved november 5 2013 james server changelog archived february 20 2020 at the wayback machine james apache org retrieved on 2013 07 17 8bitmime service advertised in response to ehlo on gmail smtp in l google com port 25 checked 23 november 2011 qmail bugs and wishlist link removed home pages de retrieved on 2013 07 17 the 8bitmime extension archived june 7 2011 at the wayback machine cr yp to retrieved on 2013 07 17 the 8bit mimetransport service extension smtp service extension for 8 bit mime transport ietf march 2011 sec 3 doi 10 17487 rfc6152 rfc 6152 hazel philip 2007 the exim smtp mail server 2nd ed cambridge uit cambridge p 418 isbn 978 0 9544529 7 5 cite book check isbn value checksum help postfix smtputf8 support is enabled by default archived august 7 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oracle com october 2017 archived from the original on november 24 2020 retrieved september 17 2020 1 2 introducing mta strict transport security mta sts hardenize blog www hardenize com archived from the original on april 25 2019 retrieved april 25 2019 starttls everywhere eff archived from the original on august 9 2019 retrieved december 4 2021 v mathavale july 21 2023 how smtp dns based authentication of named entities dane secures email communications learn microsoft com retrieved march 5 2024 implementing inbound smtp dane with dnssec for exchange online mail flow techcommunity microsoft com retrieved march 5 2024 1 2 cimpanu catalin gmail becomes first major email provider to support mta sts and tls reporting zdnet archived from the original on april 29 2019 retrieved april 25 2019 message non compliant with rfc 5322 archived from the original on january 17 2023 retrieved january 20 2021 message could not be delivered please ensure the message is rfc 5322 compliant archived from the original on january 28 2021 retrieved january 20 2021 why are the emails sent to microsoft account rejected for policy reasons archived from the original on february 14 2021 retrieved january 20 2021 references edit hughes l 1998 internet e mail protocols standards and implementation artech house publishers isbn 978 0 89006 939 4 hunt c 2003 sendmail cookbook o reilly media isbn 978 0 596 00471 2 johnson k 2000 internet email protocols a developer s guide addison wesley professional isbn 978 0 201 43288 6 loshin p 1999 essential email standards rfcs and protocols made practical john wiley sons isbn 978 0 471 34597 8 rhoton j 1999 programmer s guide to internet mail smtp pop imap and ldap elsevier isbn 978 1 55558 212 8 wood d 1999 programming internet mail o reilly isbn 978 1 56592 479 6 further reading edit rfc 1123 requirements for internet hosts application and support std 3 rfc 1870 smtp service extension for message size declaration obsoletes rfc 1653 rfc 2505 anti spam recommendations for smtp mtas bcp 30 rfc 28...
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