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the (376), mail (175), smtp (159), and (154), rfc (137), for (104), #server (93), #message (82), protocol (53), email (53), from (52), with (50), that (49), this (47), edit (39), internet (37), original (37), transfer (36), retrieved (35), archived (32), servers (32), was (30), may (29), use (29), extension (29), example (29), command (28), not (28), are (27), client (26), clients (26), can (25), which (24), data (24), messages (22), relay (22), text (21), mta (21), service (20), extensions (20), submission (19), authentication (19), address (19), support (19), other (19), access (18), used (18), tls (17), com (17), simple (16), using (16), transport (16), 2020 (16), network (16), such (16), one (16), but (16), has (15), were (15), its (15), obsoletes (14), exchange (14), bit (14), port (14), only (14), 250 (14), microsoft (13), security (13), 8bitmime (13), sender (13), information (13), user (13), esmtp (13), imap (12), 2021 (12), starttls (12), history (12), relaying (12), after 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as better suited for handling email transfers between machines that were intermittently connected smtp on the other hand works best when both the sending and receiving machines are connected to the network all the time both used a store and forward mechanism and are examples of push technology though usenet s newsgroups were still propagated with uucp between servers 16 uucp as a mail transport has virtually disappeared 17 along with the bang paths it used as message routing headers 18 sendmail released with 4 1cbsd in 1983 was one of the first mail transfer agents mta to implement smtp 19 over time as bsd unix became the most popular operating system on the internet sendmail became the most common mail transfer agent 20 the original smtp protocol supported only unauthenticated unencrypted 7 bit ascii text communications susceptible to trivial man in the middle attack spoofing and spamming and requiring any binary data to be encoded to readable text before transmission due to absence of a proper authentication mechanism by design every smtp server was an open mail relay the internet mail consortium imc reported that 55 of mail servers were open relays in 1998 21 but less than 1 in 2002 22 because of spam concerns most email providers blocklist open relays 23 making original smtp essentially impractical for general use on the internet modern smtp edit in november 1995 rfc 1869 defined extended simple mail transfer protocol esmtp which established a general structure for all existing and future extensions which aimed to add in the features missing from the original smtp esmtp defines consistent and manageable means by which esmtp clients and servers can be identified and servers can indicate supported extensions message submission rfc 2476 and smtp auth rfc 2554 were introduced in 1998 and 1999 both describing new trends in email delivery originally smtp servers were typically internal to an organization receiving mail for the organization from the outside and relaying messages from the organization to the outside but as time went on smtp servers mail transfer agents in practice were expanding their roles to become message submission agents for mail user agents some of which were now relaying mail from the outside of an organization e g a company executive wishes to send email while on a trip using the corporate smtp server this issue a consequence of the rapid expansion and popularity of the world wide web meant that smtp had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email spam work on message submission rfc 2476 was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it for example adding a domain name to an unqualified address this behavior is helpful when the message being fixed is an initial submission but dangerous and harmful when the message originated elsewhere and is being relayed cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay as spam became more prevalent it was also seen as a way to provide authorization for mail being sent out from an organization as well as traceability this separation of relay and submission quickly became a foundation for modern email security practices as this protocol started out purely ascii text based it did not deal well with binary files or characters in many non english languages standards such as multipurpose internet mail extensions mime were developed to encode binary files for transfer through smtp mail transfer agents mtas developed after sendmail also tended to be implemented 8 bit clean so that the alternate just send eight strategy could be used to transmit arbitrary text data in any 8 bit ascii like character encoding via smtp mojibake was still a problem due to differing character set mappings between vendors although the email addresses themselves still allowed only ascii 8 bit clean mtas today tend to support the 8bitmime extension permitting some binary files to be transmitted almost as easily as plain text limits on line length and permitted octet values still apply so that mime encoding is needed for most non text data and some text formats in 2012 the smtputf8 extension was created to support utf 8 text allowing international content and addresses in non latin scripts like cyrillic or chinese many people contributed to the core smtp specifications among them jon postel eric allman dave crocker ned freed randall gellens john klensin and keith moore mail processing model edit blue arrows depict implementation of smtp variations email is submitted by a mail client mail user agent mua to a mail server mail submission agent msa using smtp on tcp port 465 or 587 most mailbox providers still allow submission on traditional port 25 the msa delivers the mail to its mail transfer agent mta often these two agents are instances of the same software launched with different options on the same machine local processing can be done either on a single machine or split among multiple machines mail agent processes on one machine can share files but if processing is on multiple machines they transfer messages between each other using smtp where each machine is configured to use the next machine as a smart host each process is an mta an smtp server in its own right the boundary mta uses dns to look up the mx mail exchanger record for the recipient s domain the part of the email address on the right of the mx record contains the name of the target mta based on the target host and other factors the sending mta selects a recipient server and connects to it to complete the mail exchange message transfer can occur in a single connection between two mtas or in a series of hops through intermediary systems a receiving smtp server may be the ultimate destination an intermediate relay that is it stores and forwards the message or a gateway that is it may forward the message using some protocol other than smtp per rfc 5321 section 2 1 each hop is a formal handoff of responsibility for the message whereby the receiving server must either deliver the message or properly report the failure to do so once the final hop accepts the incoming message it hands it to a mail delivery agent mda for local delivery an mda saves messages in the relevant mailbox format as with sending this reception can be done using one or multiple computers but in the diagram above the mda is depicted as one box near the mail exchanger box an mda may deliver messages directly to storage or forward them over a network using smtp or other protocol such as local mail transfer protocol lmtp a derivative of smtp designed for this purpose once delivered to the local mail server the mail is stored for batch retrieval by authenticated mail clients muas mail is retrieved by end user applications called email clients using internet message access protocol imap a protocol that both facilitates access to mail and manages stored mail or the post office protocol pop which typically uses the traditional mbox mail file format or a proprietary system such as microsoft exchange outlook or lotus notes domino webmail clients may use either method but the retrieval protocol is often not a formal standard smtp defines message transport not the message content thus it defines the mail envelope and its parameters such as the envelope sender but not the header except trace information nor the body of the message itself std 10 and rfc 5321 define smtp the envelope while std 11 and rfc 5322 define the message header and body formally referred to as the internet message format protocol overview edit smtp is a connection oriented text based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel typically a transmission control protocol tcp connection an smtp session consists of commands originated by an smtp client the initiating agent sender or transmitter and corresponding responses from the smtp server the listening agent or receiver so that the session is opened and session parameters are exchanged a session may include zero or more smtp transactions an smtp transaction consists of three command reply sequences mail command to establish the return address also called return path 24 reverse path 25 bounce address mfrom or envelope sender rcpt command to establish a recipient of the message this command can be issued multiple times one for each recipient these addresses are also part of the envelope data to signal the beginning of the message text the content of the message as opposed to its envelope it consists of a message header and a message body separated by an empty line data is actually a group of commands and the server replies twice once to the data command itself to acknowledge that it is ready to receive the text and the second time after the end of data sequence to either accept or reject the entire message besides the intermediate reply for data each server s reply can be either positive 2xx reply codes or negative negative replies can be permanent 5xx codes or transient 4xx codes a reject is a permanent failure and the client should send a bounce message to the server it received it from a drop is a positive response followed by message discard rather than delivery the initiating host the smtp client can be either an end user s email client functionally identified as a mail user agent mua or a relay server s mail transfer agent mta that is an smtp server acting as an smtp client in the relevant session in order to relay mail fully capable smtp servers maintain queues of messages for retrying message transmissions that resulted in transient failures a mua knows the outgoing mail smtp server from its configuration a relay server typically determines which server to connect to by looking up the mx mail exchange dns resource record for each recipient s domain name if no mx record is found a conformant relaying server not all are instead looks up the a record relay servers can also be configured to use a smart host a relay server initiates a tcp connection to the server on the well known port for smtp port 25 or for connecting to an msa port 465 or 587 the main difference between an mta and an msa is that connecting to an msa requires smtp authentication smtp vs mail retrieval edit smtp is a delivery protocol only in normal use mail is pushed to a destination mail server or next hop mail server as it arrives mail is routed based on the destination server not the individual user s to which it is addressed other protocols such as the post office protocol pop and the internet message access protocol imap are specifically designed for use by individual users retrieving messages and managing mailboxes to permit an intermittently connected mail server to pull messages from a remote server on demand smtp has a feature to initiate mail queue processing on a remote server see remote message queue starting below pop and imap are unsuitable protocols for relaying mail by intermittently connected machines they are designed to operate after final delivery when information critical to the correct operation of mail relay the mail envelope has been removed remote message queue starting edit remote message queue starting enables a remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending a corresponding command the original turn command was deemed insecure and was extended in rfc 1985 with the etrn command which operates more securely using an authentication method based on domain name system information 26 outgoing mail smtp server edit an email client needs to know the ip address of its initial smtp server and this has to be given as part of its configuration usually given as a dns name this server will deliver outgoing messages on behalf of the user outgoing mail server access restrictions edit server administrators need to impose some control on which clients can use the server this enables them to deal with abuse for example spam two solutions have been in common use in the past many systems imposed usage restrictions by the location of the client only permitting usage by clients whose ip address is one that the server administrators control usage from any other client ip address is disallowed modern smtp servers typically offer an alternative system that requires authentication of clients by credentials before allowing access restricting access by location edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed october 2025 learn how and when to remove this message under this system an internet service provider s isp s smtp server will not allow access by users who are outside the isp s network more precisely the server may only allow access to users with an ip address provided by the isp which is equivalent to requiring that they are connected to the internet using that same isp a mobile user may often be on a network other than that of their normal isp and will then find that sending email fails because the configured smtp server choice is no longer accessible this system has several variations for example an organisation s smtp server may only provide service to users on the same network enforcing this by firewalling to block access by users on the wider internet or the server may perform range checks on the client s ip address these methods were typically used by corporations and institutions such as universities which provided an smtp server for outbound mail only for use internally within the organisation however most of these bodies now use client authentication methods as described below where a user is mobile and may use different isps to connect to the internet this kind of usage restriction is onerous and altering the configured outbound email smtp server address is impractical it is highly desirable to be able to use email client configuration information that does not need to change client authentication edit modern smtp servers typically require authentication of clients by credentials before allowing access rather than restricting access by location as described earlier this more flexible system is friendly to mobile users and allows them to have a fixed choice of configured outbound smtp server smtp authentication often abbreviated smtp auth is an extension of the smtp in order to log in using an authentication mechanism ports edit communication between mail servers generally uses the standard tcp port 25 designated for smtp mail clients however generally don t use this instead using specific submission ports mail services generally accept email submission from clients on one of 465 this port was deprecated after rfc 24...