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Postmaster reference page

This page provides technical information about Email Routing to professionals who administer email systems, and other email providers.

Here you will find information regarding Email Routing, along with best practices, rules, guidelines, troubleshooting tools, as well as known limitations for Email Routing.

​​ Postmaster

​​ Authenticated Received Chain (ARC)

Email Routing supports Authenticated Received Chain (ARC). ARC is an email authentication system designed to allow an intermediate email server (such as Email Routing) to preserve email authentication results. Google also supports ARC.

​​ Contact information

The best way to contact us is using our community forum or our Discord server.

​​ DKIM signature

DKIM (DomainKeys Identified Mail) ensures that email messages are not altered in transit between the sender and the recipient’s SMTP servers through public-key cryptography.

Through this standard, the sender publishes its public key to a domain’s DNS once, and then signs the body of each message before it leaves the server. The recipient server reads the message, gets the domain public key from the domain’s DNS, and validates the signature to ensure the message was not altered in transit.

Email Routing signs email on behalf of email.cloudflare.net. If the sender did not sign the email, the receiver will likely use Cloudflare’s signature for authentication.

Below is the DKIM key for email.cloudflare.net:


$ dig TXT 2022._domainkey.email.cloudflare.net +short


"v=DKIM1; h=sha256; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAnraPy1d8e6+lzeE1HIoUvYWoAOUSREkNHcwxA/ueVM8f6FKXvPu/9gVpgkn8iUyaCfk2z1MW+OVLuFeH64YRMa39mkaQalgke2tZ05SnjRUtYEHYvfrqPuMT+Ouk+GecpgvrtMq5gMXm6ZfeUhQkdWxmMQJGf4fdW5I0piUQJMhK/Qc1dNRSskk" "TiUtXKnsEdjTN2xcnHhyj985S0xOEAxm9Uj1rykPqVvKpqEdjUkujbXOwR0KmHTvPyFpBjCCfxAVqOwwo9zBYuvk/nh0qlDgLIpy0SimrYhNFCq2XBxIj4tdUzIl7qZ5Ck6zLCQ+rjzJ4sm/zA+Ov9kDkbcmyrwIDAQAB"

​​ DMARC enforcing

Email Routing enforces Domain-based Message Authentication, Reporting & Conformance (DMARC). Depending on the sender’s DMARC policy, Email Routing will reject emails when there is an authentication failure. Refer to dmarc.org for more information on this protocol.

​​ IPv6 support

Currently, Email Routing will connect to the upstream SMTP servers using IPv6 if they provide AAAA records for their MX servers, and fall back to IPv4 if that is not possible.

Below is an example of a popular provider that supports IPv6:


$ dig mx gmail.com
 
gmail.com. 3084 IN MX 5 gmail-smtp-in.l.google.com.
gmail.com. 3084 IN MX 20 alt2.gmail-smtp-in.l.google.com.
gmail.com. 3084 IN MX 40 alt4.gmail-smtp-in.l.google.com.
gmail.com. 3084 IN MX 10 alt1.gmail-smtp-in.l.google.com.
gmail.com. 3084 IN MX 30 alt3.gmail-smtp-in.l.google.com.
 
$ dig AAAA gmail-smtp-in.l.google.com
 
gmail-smtp-in.l.google.com. 17 IN AAAA 2a00:1450:400c:c09::1b

Email Routing also supports IPv6 through Cloudflare’s inbound MX servers.

​​ MX, SPF, and DKIM records

Email Routing automatically adds a few DNS records to the zone when our customers enable Email Routing. If we take example.com as an example:


example.com. 300 IN MX 13 amir.mx.cloudflare.net.
example.com. 300 IN MX 86 linda.mx.cloudflare.net.
example.com. 300 IN MX 24 isaac.mx.cloudflare.net.


example.com. 300 IN TXT "v=spf1 include:_spf.mx.cloudflare.net ~all"

The MX (mail exchange) records tell the Internet where the inbound servers receiving email messages for the zone are. In this case, anyone who wants to send an email to example.com can use the amir.mx.cloudflare.net, linda.mx.cloudflare.net, or isaac.mx.cloudflare.net SMTP servers.

​​ Outbound hostnames

In addition to the outbound prefixes, Email Routing will use the domain email.cloudflare.net for the HELO/EHLO command.

PTR records (reverse DNS) ensure that each hostname has an corresponding IP. For example:


$ dig a0-7.email.cloudflare.net +short
104.30.0.7


$ dig -x 104.30.0.7 +short
a0-7.email.cloudflare.net.

​​ Outbound prefixes

Email Routing sends its traffic using both IPv4 and IPv6 prefixes, when supported by the upstream SMTP server.

If you are a postmaster and are having trouble receiving Email Routing’s emails, allow the following outbound IP addresses in your server configuration:

IPv4

104.30.0.0/19

IPv6

2405:8100:c000::/38

Ranges last updated: October 4th, 2022

​​ Sender rewriting

Email Routing rewrites the SMTP envelope sender (MAIL FROM) to the forwarding domain to avoid issues with SPF. Email Routing uses the Sender Rewriting Scheme to achieve this.

This has no effect to the end user’s experience, though. The message headers will still report the original sender’s From: address.

​​ SMTP errors

In most cases, Email Routing simply forwards the upstream SMTP errors back to the sender client in-session.

​​ Spam and abusive traffic

Handling spam and abusive traffic is essential to any email provider. Email Routing filters emails based on advanced anti-spam criteria, powered by Area 1. When Email Routing detects and blocks a spam email, you will receive a message with details explaining what happened. For example:


554 <YOUR_IP_ADDRESS> found on one or more DNSBLs (abusixip). Refer to https://developers.cloudflare.com/email-routing/postmaster/#spam-and-abusive-traffic/

​​ SPF record

A SPF DNS record is an anti-spoofing mechanism that is used to specify which IP addresses and domains are allowed to send emails on behalf of your zone.

The Internet Engineering Task Force (IETF) tracks the SPFv1 specification in RFC 7208. Refer to the SPF Record Syntax to learn the SPF syntax.

Email Routing’s SPF record contains the following:


v=spf1 include:_spf.mx.cloudflare.net ~all

In the example above:

  • spf1: Refers to SPF version 1, the most common and more widely adopted version of SPF.
  • include: Include a second query to _spf.mx.cloudflare.net and allow its contents.
  • ~all: Otherwise SoftFail on all other origins. SoftFail means NOT allowed to send, but in transition. This instructs the upstream server to accept the email but mark it as suspicious if it came from any IP addresses outside of those defined in the SPF records.

If we do a TXT query to _spf.mx.cloudflare.net, we get:


_spf.mx.cloudflare.net. 300 IN TXT "v=spf1 ip4:104.30.0.0/20 ~all"

This response means:

  • Allow all IPv4 IPs coming from the 104.30.0.0/20 subnet.
  • Otherwise, SoftFail.

You can read more about SPF, DKIM, and DMARC in our Tackling Email Spoofing and Phishing blog.

​​ Known limitations

Below, you will find information regarding known limitations for Email Routing.

​​ Email address internationalization (EAI)

Email Routing does not support internationalized email addresses. Email Routing only supports internationalized domain names.

This means that you can have email addresses with an internationalized domain, but not an internationalized local-part (the first part of your email address, before the @ symbol). Refer to the following examples:

  • info@piñata.es - Supported.
  • piñata@piñata.es - Not supported.

​​ Non-delivery reports (NDRs)

Email Routing does not forward non-delivery reports to the original sender. This means the sender will not receive a notification indicating that the email did not reach the intended destination.

​​ Restrictive DMARC policies can make forwarded emails fail

Due to the nature of email forwarding, restrictive DMARC policies might make forwarded emails fail to be delivered. Refer to dmarc.org for more information.

​​ Sending or replying to an email from your Cloudflare domain

Email Routing does not support sending or replying from your Cloudflare domain. When you reply to emails forwarded by Email Routing, the reply will be sent from your destination address (like my-name@gmail.com), not your custom address (like info@my-company.com).

​​ Signs such “+” and “.” are treated as normal characters for custom addresses

Email Routing does not have advanced routing options. Characters such as + or ., which perform special actions in email providers like Gmail and Outlook, are currently treated as normal characters on custom addresses. More flexible routing options are in our roadmap.

​​ Subdomains are only supported for Enterprise customers

Subdomains cannot use Email Routing to forward emails, unless they are part of an Enterprise account.