Best practices for sender authentication support in Azure Communication Services Email

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This article provides the Email Sending best practices on DNS records and how to use the sender authentication methods that help prevent attackers from sending messages that look like they come from your domain.

Email authentication and DNS setup

Sending an email requires several steps which include verifying the sender of the email actually owns the domain, checking the domain reputation, virus scanning, filtering for spam, phishing attempts, malware etc. Configuring proper email authentication is a foundational principle for establishing trust in email and protecting your domain’s reputation. If an email passes authentication checks, the receiving domain can apply policy to that email in keeping with the reputation already established for the identities associated with those authentication checks, and the recipient can be assured that those identities are valid.

MX (Mail Exchange) record

MX (Mail Exchange) record is used to route email to the correct server. It specifies the mail server responsible for accepting email messages on behalf of your domain. DNS needs to be updated with the latest information of MX records of your email domain otherwise it will result in some delivery failures.

SPF (Sender Policy Framework)

SPF RFC 7208 is a mechanism that allows domain owners to publish and maintain, via a standard DNS TXT record, a list of systems authorized to send email on their behalf. This record is used to specify which mail servers are authorized to send email on behalf of your domain. It helps to prevent email spoofing and increase email deliverability.

DKIM (Domain Keys Identified Mail)

DKIM RFC 6376 allows an organization to claim responsibility for transmitting a message in a way that can be validated by the recipient. This record is also used to authenticate the domain the email is sent from, and helps to prevent email spoofing and increase email deliverability.

DMARC (Domain-based Message Authentication, Reporting, and Conformance)

DMARC RFC 7489 is a scalable mechanism by which a mail-originating organization can express domain-level policies and preferences for message validation, disposition, and reporting that a mail-receiving organization can use to improve mail handling. It is also used to specify how email receivers should handle messages that fail SPF and DKIM checks. This improves email deliverability and helps to prevent email spoofing.

ARC (Authenticated Received Chain)

The ARC protocol RFC 8617 provides an authenticated chain of custody for a message, allowing each entity that handles the message to identify what entities handled it previously as well as the message’s authentication assessment at each hop. ARC is not yet an internet standard, but adoption is increasing.

How Email authentication works

Email authentication verifies that email messages from a sender (for example, notification@contoso.com) are legitimate and come from expected sources for that email domain (for example, contoso.com.) An email message may contain multiple originator or sender addresses. These addresses are used for different purposes. For example, consider these addresses:

  • Mail From address identifies the sender and specifies where to send return notices if any problems occur with the delivery of the message, such as non-delivery notices. This appears in the envelope portion of an email message and is not displayed by your email application. This is sometimes called the 5321.MailFrom address or the reverse-path address.

  • From address is the address displayed as the From address by your mail application. This address identifies the author of the email. That is, the mailbox of the person or system responsible for writing the message. This is sometimes called the 5322.From address.

  • Sender Policy Framework (SPF) helps validate outbound email sent from your mail from domain (is coming from who it says it is).

  • DomainKeys Identified Mail (DKIM) helps to ensure that destination email systems trust messages sent outbound from your mail from domain.

  • Domain-based Message Authentication, Reporting, and Conformance(DMARC) works with Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) to authenticate mail senders and ensure that destination email systems trust messages sent from your domain.

Implementing DMARC

Implementing DMARC with SPF and DKIM provides rich protection against spoofing and phishing email. SPF uses a DNS TXT record to provide a list of authorized sending IP addresses for a given domain. Normally, SPF checks are only performed against the 5321.MailFrom address. This means that the 5322.From address is not authenticated when you use SPF by itself. This allows for a scenario where a user can receive a message, which passes an SPF check but has a spoofed 5322.From sender address.

Like the DNS records for SPF, the record for DMARC is a DNS text (TXT) record that helps prevent spoofing and phishing. You publish DMARC TXT records in DNS. DMARC TXT records validate the origin of email messages by verifying the IP address of an email's author against the alleged owner of the sending domain. The DMARC TXT record identifies authorized outbound email servers. Destination email systems can then verify that messages they receive originate from authorized outbound email servers. This forces a mismatch between the 5321.MailFrom and the 5322.From addresses in all email sent from your domain and DMARC will fail for that email. To avoid this, you need to set up DKIM for your domain.

A DMARC policy record allows a domain to announce that their email uses authentication; provides an email address to gather feedback about the use of their domain; and specifies a requested policy for the handling of messages that do not pass authentication checks. We recommend that

  • Policy statements domains publishing DMARC records be “p=reject” where possible, “p=quarantine” otherwise.
  • The policy statement of “p=none”, “sp=none”, and pct<100 should only be viewed as transitional states, with the goal of removing them as quickly as possible.
  • Any published DMARC policy record should include, at a minimum, a "rua" tag that points to a mailbox for receiving DMARC aggregate reports and should send no replies back when receiving reports due to privacy concerns.

Next steps

The following documents may be interesting to you: