Learning to find the source owner using the ‘envelope_to’ domain

An ‘envelope_to’ domain is the domain in the recipient’s email address. This address can be used to understand how some of the legitimate emails are failing DMARC checks. If you are unsure about the troubleshooting methods, using the ‘envelope_to’ domain to find the source owner is a wise choice. 

Using this technique, you get to know which third-party service providers are sending emails on your behalf and to whom. You can also trace incoming emails in your logs to know which employee is using which source to send official emails. If you spot an unauthorized source, you can take the requisite action before it becomes an exploitable vulnerability.

How to match sending sources with internal records?

It’s important that you compare the sending sources with the internal records so that you know about the use of shadow IT and unauthorized mail servers being used in your company. This practice also helps bring misconfigurations to the surface. Here’s how you can match and verify-

Check against the list of officially authorized senders

DMARC is based on SPF results, and for SPF deployment, you need to create and upload a list of authorized senders on your DNS. So, pull out the same list and check if the sending sources align with your records. If you find any unauthorized source, investigate further. 

Cross-check with internal email infrastructure

Verify that sending sources (IPs, domains) align with your organization’s email gateways, servers, and internal applications. If an email appears to originate from an internal department but is sent from an unrecognized server, it could signal spoofing or a configuration issue.

Correlate with employees or team activity

If an email is sent from your domain but the source isn’t authorized, consult your internal team. It could be an unauthorized sender, a former employee’s access still in use, or a misconfigured application unintentionally sending emails.

Discovering source owners strengthens email security

Detecting an unapproved sender while discovering source owners using the ‘envelope_to’ domain indicates some misconfigurations or security gaps. Pay immediate attention to these so that a threat actor can’t exploit them to send phishing and spoofing emails on your behalf. Always opt-in to receive RUA and RUF reports, as they provide insights into your email activities, helping SPF, DKIM, and DMARC be highly efficient in securing emails. 

If you monitor DMARC reports properly, you will also know if your DMARC record requires any adjustments in the policy-

• The ‘none’ policy instructs the receiving server to take no action against emails that failed the DMARC checks. Such emails are treated normally.
• The ‘quarantine’ policy instructs the receiving server to place such emails in the spam folder and treat them with caution because they are potentially risky.
• The ‘reject’ policy instructs the receiving server to outrightly reject the entry of emails that don’t pass the authentication checks. Such emails bounce back to the sender, making p=reject the strictest policy. 

Also, with proper DMARC management, you stay compliant with industry standards, including GDPR, DORA, and HIPAA.