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A "Basic Status Code" SMTP reply consists of a three digit number (transmitted as three numeric characters) followed by some text. The number is for use by automata (e.g., email clients) to determine what state to enter next; the text ("Text Part") is for the human user. The first digit denotes whether the response is good, bad, or incomplete:
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 ...
Callback verification, also known as callout verification or Sender Address Verification, is a technique used by SMTP software in order to validate e-mail addresses. The most common target of verification is the sender address from the message envelope (the address specified during the SMTP dialogue as " MAIL FROM ").
A non-standard but widely used way to request return receipts is with the "Return-Receipt-To:" (RRT) field in the e-mail header, with the email return address specified. The first time a user opens an email message containing this field in the header, the client will typically prompt the user whether to send a return receipt.
On the other hand, UTF-16 must be transfer encoded to fit SMTP data format. Although not strictly required, UTF-8 is usually also transfer encoded to avoid problems across seven-bit mail servers. MIME transfer encoding of UTF-8 makes it either unreadable as a plain text (in the case of base64 ) or, for some languages and types of text, heavily ...
SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol (SMTP) whereby a client may log in using any authentication mechanism supported by the server. It is mainly used by submission servers, where authentication is mandatory.
Similarly, some e-mail systems use unique per-message return-paths, for example variable envelope return path (VERP) for mailing lists, Sender Rewriting Scheme for forwarded e-mail, Bounce Address Tag Validation for backscatter protection, etc. If an exact match on the sender address is required, every e-mail from such systems will be delayed.
In times when both mail submission and acceptance of incoming mail were usually accomplished using the same protocol and the same server, the ability to send mail to arbitrary destinations without authentication allowed spammers to use MTAs as a means of distributing spam (since a single message transaction can request that an MTA relay a ...