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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
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: