Mail Exchanges

DNS is the internet’s telephone directory. When you to browse to example.com, the browser looks up “example.com” against DNS servers, and the DNS server returns the address of example.com in a format that a browser can use to display the requested page. These requests are called A record DNS queries because the browser requests a page’s address.

However, there are different types of resources that are available over the internet. A highly requested resource is email. To send an email, your mail server needs to know the address of the server handling the receipient’s email. These DNS queries are MX (“mail exchanger”) requests.

In a most organisations, staff use the organisation’s mail service to send out email. There are good reasons for centralising email handling:
1.enforcing standards like templates and attachment sizes
2.archiving email for complaince purposes
3.scanning email for security threats

There are very few good reasons why email should not be sent via the central mail server. Indeed, MX requests from individual machines is highly unusual.
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You’ve Sent Mail

Individual machines trying to contact MX servers directly usually indicates a security issue. Malware on the machine may be trying to send phishing email to external victims, sending email spam, or acting in concert with other infected machines as part of a DDOS attack.

The new Rawstream reporting helps identify suspicious MX DNS requests. The report shows the looked up domains, the number of lookups for that domain, the domain’s category, and the country where the domain is hosted.