Open mail relay
An open mail relay is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users.
SMTP Response Codes
• 2xx response codes.
These codes mane that everything is OK, go ahead with the next command.
For example after every successful command the receiving server will respond
with a 250 response code.
• 3xx response codes
These are informational codes. For example, after sending the command
DATA the receiving server will respond with a information message to start
data input.
• 4xx response codes
These responses indicate a temporary problem, try again later. This is also
known as a deferral or temp fail.
• 5xx response codes
These indicate a permanent error, and the connection is dropped. This is also
known as a rejection or a perm-fail.
Postfix
Postfix is a free and open source mail transfer agent (MTA), a computer program for the routing and delivery of email. It is intended as a fast, easy-to-administer, and secure alternative to the widely-used Sendmail MTA.
Sendmail
Sendmail is a mail transfer agent (MTA) that supports many kinds of mail transfer and delivery including the overwhelmingly popular SMTP.
MIME(Multipurpose Internet Mail Extensions)
MIME is an Internet standard that extends the format of e-mail to support:
1, Text in character sets other than ASCII
2, Non-text attachments
3, Message bodies with multiple parts
4, Header information in non-ASCII character sets
SMTP commands
Helo / ehlo States who it is (its hostname)
MAIL From States who the sender is (Envelope From:)
RCPT To States where the message is goi ng (Envelope Recipient (s)
Data Transmits content of the message, followed by Headers
Quit Terminates connection
How to register a mail server in DNS server
1. Create A record that maps hostname of appliance to IP address
2. Create MX record that maps public domain to appliance hostname
3. Specify priority for MX record to advertise appliance as primary or backup (The smallest preference number has the highest priority)
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