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dized in rfc 2645 that allows an intermittently connected smtp server to receive email queued for it when it is connected internationalization extension edit main article international email original smtp supports email addresses composed of ascii characters only which is inconvenient for users whose native script is not latin based or who use diacritic not in the ascii character set this limitation was alleviated via extensions enabling utf 8 in address names rfc 5336 introduced experimental 32 utf8smtp command and later was superseded by rfc 6531 that introduced smtputf8 command these extensions provide support for multi byte and non ascii characters in email addresses such as those with diacritics and other language characters such as greek and chinese 34 current support is limited but there is strong interest in broad adoption of rfc 6531 and the related rfcs in countries like china that have a large user base where latin ascii is a foreign script extensions edit like smtp esmtp is a protocol used to transport internet mail it is used as both an inter server transport protocol and with restricted behavior enforced a mail submission protocol the main identification feature for esmtp clients is to open a transmission with the command ehlo extended hello rather than helo hello the original rfc 821 standard a server will respond with success code 250 failure code 550 or error code 500 501 502 504 or 421 depending on its configuration an esmtp server returns the code 250 ok in a multi line reply with its domain and a list of keywords to indicate supported extensions a rfc 821 compliant server returns error code 500 allowing esmtp clients to try either helo or quit each service extension is defined in an approved format in subsequent rfcs and registered with the internet assigned numbers authority iana the first definitions were the rfc 821 optional services send soml send or mail saml send and mail expn help and turn the format of additional smtp verbs was set and for new parameters in mail and rcpt some relatively common keywords not all of them corresponding to commands used today are 8bitmime 8 bit data transmission rfc 6152 atrn authenticated turn for on demand mail relay rfc 2645 auth authenticated smtp rfc 4954 chunking chunking rfc 3030 dsn delivery status notification rfc 3461 see variable envelope return path etrn extended version of remote message queue starting command turn rfc 1985 help supply helpful information rfc 821 pipelining command pipelining rfc 2920 size message size declaration rfc 1870 starttls transport layer security rfc 3207 2002 smtputf8 allow utf 8 encoding in mailbox names and header fields rfc 6531 utf8smtp allow utf 8 encoding in mailbox names and header fields rfc 5336 deprecated 35 the esmtp format was restated in rfc 2821 superseding rfc 821 and updated to the latest definition in rfc 5321 in 2008 support for the ehlo command in servers became mandatory and helo designated a required fallback non standard unregistered service extensions can be used by bilateral agreement these services are indicated by an ehlo message keyword starting with x and with any additional parameters or verbs similarly marked smtp commands are case insensitive they are presented here in capitalized form for emphasis only an smtp server that requires a specific capitalization method is a violation of the standard 28 8bitmime edit at least the following servers advertise the 8bitmime extension apache james since 2 3 0a1 36 citadel since 7 30 courier mail server gmail 37 icewarp iis smtp service kerio connect lotus domino microsoft exchange server as of exchange server 2000 novell groupwise opensmtpd oracle communications messaging server postfix sendmail since 6 57 zmailer since 2 95 940901 the following servers can be configured to advertise 8bitmime but do not perform conversion of 8 bit data to 7 bit when connecting to non 8bitmime relays exim and qmail do not translate eight bit messages to seven bit when making an attempt to relay 8 bit data to non 8bitmime peers as is required by the rfc 38 this does not cause problems in practice since virtually all modern mail relays are 8 bit clean 39 microsoft exchange server 2003 advertises 8bitmime by default but relaying to 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 partridge 2008 the technical development of internet email pdf ieee annals of the history of computing vol 30 ieee computer society pp 3 29 bibcode 2008iahc 30b 3p doi 10 1109 mahc 2008 32 s2cid 206442868 archived from the original pdf on may 12 2011 paul hoffman february 1 1998 allowing relaying in smtp a survey internet mail consortium archived from the original on march 5 2016 retrieved may 30 2010 paul hoffman august 2002 allowing relaying in smtp a series of surveys internet mail consortium archived from the original on january 18 2007 retrieved may 30 2010 in unix what is an open mail relay knowledge base june 17 2007 archived from the original on june 17 2007 retrieved march 15 2021 the mail rcpt and data verbs archived february 22 2014 at the wayback machine d j bernstein rfc 5321 section 7 2 systems message message systems introduces latest version of momentum with new api driven capabilities www prnewswire com press release archived from the original on july 19 2020 retrieved july 19 2020 cara garretson 2005 isps pitch in to stop spam pc world archived from the original on august 28 2015 retrieved january 18 2016 last month the anti spam technical alliance formed last year by yahoo america online earthlink and microsoft issued a list of antispam recommendations that includes filtering port 25 1 2 j klensin october 2008 simple mail transfer protocol ietf network working group doi 10 17487 rfc5321 rfc 5321 draft standard updated by rfc 7504 obsoletes rfc 2821 updates rfc 1123 rfc 1047 klensin john c october 2008 data termination 10 minutes simple mail transfer protocol ietf sec 4 5 3 2 6 doi 10 17487 rfc5321 rfc 5321 retrieved june 7 2010 john klensin ned freed marshall t rose einar a stefferud dave crocker november 1995 smtp service extensions...
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