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PHPMailer is a code library to send (transport) emails safely [8] and easily via PHP code from a web server (MUA to the MSA server). Sending emails directly by PHP code requires a high-level familiarity to SMTP protocol standards (RFC 821, 2821, 5321) and related issues (such as Carriage return) and vulnerabilities about email injection for ...
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:
This may also cause each e-mail to be delayed unless the greylisting system ignores the tag, or whitelists sending hosts that successfully retry. Challenge-response spam filtering and systems that sort mail based on the bounce address (e.g. for removing duplicates) may work less smoothly with BATV-tagged addresses.
In the early 1980s, when Simple Mail Transfer Protocol (SMTP) was designed, it provided for no real verification of sending user or system. This was not a problem while email systems were run by trusted corporations and universities, but since the commercialization of the Internet in the early 1990s, spam, phishing, and other crimes have been found to increasingly involve email.
Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and if sufficient time has elapsed, the email will be accepted.
Sender Policy Framework (SPF) is an email authentication method that ensures the sending mail server is authorized to originate mail from the email sender's domain. [1] [2] This authentication only applies to the email sender listed in the "envelope from" field during the initial SMTP connection.
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SPF checks that the IP address of the sending server is authorized by the owner of the domain that appears in the SMTP MAIL FROM command. (The email address in MAIL FROM is also called the bounce address, envelope-from or RFC5321.MailFrom.) In addition to requiring that the SPF check passes, DMARC checks that RFC5321.MailFrom aligns with 5322.From.