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How to successfully deliver messages ?

Adhere to the standards

Make sure emails sent by your server are RFC compliant:

Use proper reverse DNS

  • The lack of reverse DNS will impact your deliverability.
  • Having a valid forward, non-generic PTR record helps (see FCrDNS).
  • Dynamic IP space is a common haven for compromised machines involved in bot networks, use a static mail server hostname instead.

Stay off blocklists

  • Several DNSBLs are used to filter out known bad senders.
  • Being listed in DNSBLs harms not only here, but at other ISPs as well.

Authenticate your email

  • Implement the standard SMTP authentication mechanisms:

Send messages your users want only

  • Require subscribers to your mailing list to opt-in.
    • Let them know the frequency of your communication.
    • Respect that frequency!
  • Provide an easy unsubscribe process to your users.
  • Do NOT purchase mailing lists 🚩

Keep an eye on your email flow

  • Remove invalid users:
    • Monitor responses to RCPT TO commands
    • Care about the bounces you receive
  • Restrictions on incoming connections and messages exist, abusers are (at least) rate-limited.

What if I am blocked ?

Temporary errors

SMTP responses starting with a 4XX code indicate a temporary issue.
There are multiple reasons why such a response can be returned but since it is a temporary problem, retrying later might work.

Permanent errors

SMTP responses starting with a 5XX code indicate a permanent issue.
Multiple reasons can cause such a response and senders receiving this kind of error should not retry.
Moreover, lists' owners with users generating 5xx errors (and/or bounces) should consider removing them as part of list hygiene.

Review the SMTP error codes

SMTP responses we return are explained in the following sections and sometimes include a remediation link: