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In this example the email message is sent to two mailboxes on the same SMTP server: one for each recipient listed in the To: and Cc: header fields. The corresponding SMTP command is RCPT TO . Each successful reception and execution of a command is acknowledged by the server with a result code and response message (e.g., 250 Ok ).
C: This is a test message with 5 header fields and 4 lines in the message body. C: Your friend, C: Bob C: . S: 250 Ok: queued as 12345 C: QUIT S: 221 Bye {The server closes the connection} And below is an example of an SMTP connection in which the SMTP Server supports the Enhanced Status Code, taken from RFC 2034:
In order to prevent being blacklisted, major free email services such as Gmail, Yahoo, and Hotmail, as well as major ISPs now implement outgoing anti-spam countermeasures. Gmail, for example, continues to get listed and delisted [9] [10] because they refuse abuse reports. [11] However, smaller networks may still be unwittingly blocked.
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.
A message submission agent (MSA), or mail submission agent, is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP), as specified in RFC 6409. [1]
Messages from non-local IP addresses to local mailboxes; Messages from clients that are authenticated and authorized; In particular, a properly secured SMTP mail relay should not accept and forward arbitrary e-mails from non-local IP addresses to non-local mailboxes by an unauthenticated or unauthorized user.
Email filtering is the processing of email to organize it according to specified criteria. The term can apply to the intervention of human intelligence, but most often refers to the automatic processing of messages at an SMTP server, possibly applying anti-spam techniques. Filtering can be applied to incoming emails as well as to outgoing ones.
SMTP proxies often implement the first and/or only layer of defence in an inbound anti-spam filtering system, where they can analyze messages using a spam content filter or antivirus program, block or rate limit connections using DNS blacklists and reputation systems, and load-balance SMTP connections to prevent overloading of mail servers.