This is a simple how to restrict how to achieve a lot of SPAM, this will make use of the utilities' own Postfix.

The problem with most spam or junk mail, is that these emails come from invalid addresses or domains. With that said, using smtpd_helo_restrictions we fought for over half of SPAM, as if the helo is invalid, there is no similar or rejects it.

If you already enter smtpd_sender_restrictions restrictions, we can see that there is mail from el@dominio.com of several checks and more.

smtpd_sender_restrictions:

reject_unknown_sender_domain -> It de@dominio.com a resolution and if there is no mail undone.

reject_non_fqdn_sender -> Requires a FQDN

smtpd_helo_restrictions:

reject_invalid_hostname -> You have to be valid

reject_unknown_hostname -> helo That has to exist

reject_non_fqdn_hostname -> It must be FQDN

Because Postfix eg $ myhostname salutes that can be localhost.localdomain (OLEE) or mail.guatewireless.org, for example, there will be a mail.guatewireless.org TO HOST pointing to an IP.

Blacklists

These lists provide another way to achieve block SPAM, the problem of using such lists is that by restricting directly addresses these sometimes renewing and are used by another person, therefore, be on the blacklist.

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