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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:
When you get a message from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject similar to "Failed Delivery," this means that an email you sent was undeliverable and has been bounced back to you. These messages are sent automatically and often include the reason for the delivery failure.
This class of status code indicates the client must take additional action to complete the request. Many of these status codes are used in URL redirection. [2]A user agent may carry out the additional action with no user interaction only if the method used in the second request is GET or HEAD.
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 ...
Although the SMTP is a mature technology, counting more than thirty years, its architecture is increasingly strained by both normal and unsolicited load. [2] The email systems have been enhanced with reputation systems tied to the actual sender of the email, with the idea of recipient's email servers rejecting the email when a forged sender is used in the protocol.
If the user's IP address is not included in the list of permitted addresses, a 403 status code is returned. Server configuration: The server's configuration can be set to prohibit access to certain files, directories, or areas of the website. This can be due to a misconfiguration or intentional restrictions imposed by the server administrator.
OpenSMTPD (also known as OpenBSD SMTP Server) is a Unix daemon implementing the Simple Mail Transfer Protocol to deliver messages on a local machine or to relay them to other SMTP servers. It was publicly released on 17 March 2013 with version number 5.3, after being in development since late 2008.
The server policy requires an Identity header, and one has not been provided. [14]: p11 429 Provide Referrer Identity The server did not receive a valid Referred-By token on the request. [15] 430 Flow Failed A specific flow to a user agent has failed, although other flows may succeed.