SMTP for mail clients

Install the Easy WP SMTP plugin

SMTP Authentication
Note If there is an email transmission in progress and you call SendAsync or Send again, you will receive an InvalidOperationException. After talking to godaddy they mentioned that my logo image in my signature was throwing a flag and marking it as spam, or gmail was, I have no idea. Server administrators need to impose some control on which clients can use the server. Users whose native script is not Latin based, or who use diacritic not in the ASCII character set have had difficulty with the Latin email address requirement. To include an attachment with an email message, first create the attachment by using the Attachment class, and then add it to the message by using the MailMessage. Within 2 minutes I had automated a smtp script that sent email to whomever or how ever many recipients I wanted. Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII.

Navigation menu

SMTP Mail Sender

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. 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. 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. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.

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. 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. 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.

Users whose native script is not Latin based, or who use diacritic not in the ASCII character set have had difficulty with the Latin email address requirement.

This server will deliver outgoing messages on behalf of the user. 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:.

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. 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. A server that is accessible on the wider Internet and does not enforce these kinds of access restrictions is known as an open relay.

This is now generally considered a bad practice worthy of blacklisting. Mail clients however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of:. Port and others may be used by some individual providers, but have never been officially supported. Most Internet service providers now block all outgoing port 25 traffic from their customers as an anti-spam measure. A typical example of sending a message via SMTP to two mailboxes alice and theboss located in the same mail domain example.

In this example, the conversation parts are prefixed with S: After the message sender SMTP client establishes a reliable communications channel to the message receiver SMTP server , the session is opened with a greeting by the server, usually containing its fully qualified domain name FQDN , in this case smtp.

The client initiates its dialog by responding with a HELO command identifying itself in the command's parameter with its FQDN or an address literal if none is available. This is also the return or bounce address in case the message cannot be delivered. In this example the email message is sent to two mailboxes on the same SMTP server: Each successful reception and execution of a command is acknowledged by the server with a result code and response message e.

The transmission of the body of the mail message is initiated with a DATA command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. Since a message body can contain a line with just a period as part of the text, the client sends two periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called dot-stuffing.

The server's positive reply to the end-of-data, as exemplified, implies that the server has taken the responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e. Until the sender has received that reply, it must assume the message was not delivered.

On the other hand, after the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of the message that they will try to deliver. The limiting timeout is specified to be 10 minutes. The QUIT command ends the session. If the email has other recipients located elsewhere, the client would QUIT and connect to an appropriate SMTP server for subsequent recipients after the current destination s had been queued.

It adds a Received and Return-Path header field, respectively. Some clients are implemented to close the connection after the message is accepted Ok: This causes an error on the server when trying to send the reply. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute.

Depending on the server's actual resource usage, it may be currently unable to accept a message this large. MSA usage is supported by most software [7] and is recommended, especially to support nomadic users, as several network hubs either block port 25 or use SMTP proxies. The MSA is responsible for ensuring that the message envelope contains good addresses, and may enforce local policies for the From header field.

Verifying that the envelope sender a. For example, from RFC The specification recommends that servers issue 5. Only servers listening on port , or private servers, should be configured that way, not a Message eXchange MX.

That way, a sender can identify itself and transmit several messages during the same session. While the authentication doesn't need to vary, once established, different messages may be sent according to different agreements and hence require different authorization.

For example, messages may be relayed on behalf of different users. Use of this parameter is much less popular than using the command to grant relay privileges. From Wikipedia, the free encyclopedia.