When a sending server makes a connection to the recipient server, the recipient server notes the sending IP address and performs a reverse lookup, called a PTR lookup, named after the type of DNS record used. If the result of the reverse lookup matches the result of a forward DNS Lookup, then it's much more likely that the message is legitimate. If the IP address doesn't match, it's much more likely that the sending address was spoofed and therefore much more likely that it's unwanted and could be considered spam.
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MX servers are accessibles
Smtp servers that are listed in DNS area must be accessible, otherwise, there is a risk that emails may be lost
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According to the RFC 2181, smtp server should accept HELO command
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According to the RFC 2181, smtp server should accept EHLO command
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STARTTLS command is accepted
STARTTLS turns an unencrypted connection into a secure connection. Note: You can use a service like altospam to solve this problem. Click on the link below to learn more
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EXPN command is now considered to be a security risk, spammers being able to harvest valid e-mail addresses via each mailing list.
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As the EXPN command, VRFY is used by spammers to verify an address.
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MX servers are not open relay
If a server is open relay, there is a risk that spammers use your server to send illegitimate mail.
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MX servers accept abuse@ address
According to the RFC 2142, SMTP server should accept abuse@yourDomain as a recipient.
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MX servers accept postmaster@ address
According to the RFC 5321, SMTP server should accept postmaster@yourDomain as a recipient.
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Banner return 2xx or 4xx code
The banner must return (2xx) a valid (4xx) or temporary code.
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Banner return server name
Banner must contain the name of the server
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SMTP server type is hidden
There is a risk to display the type and the version of the server, because people can find a breach for a specific version and use it
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