So you want to decode email headers

So you’re looking to boost your email game, get your hands dirty and stay in the running to be the world’s best email sender — or something like that. This post will give you a practical understanding of email headers, their importance, how to access them and how to read them.

Why should you read on?

Email headers provide rich information to help with any number of investigations. Most importantly, email headers provide a means of verifying the origin of a message. Other cool beans things headers help with are;

• Authentication results (SPF/DKIM). These are critical to solid deliverability!

• Any hops the message may have taken along the way

• Timestamps from when the message reached certain waypoints

Anyone working in the following areas should know the basics about email headers:

• Developers

• Marketers

• Security/Abuse teams

• Fellow dorm-room nerds (I thought you looked familiar!)

Step 1: Get to the Choppa! (e.g. head on over to your personal inbox)
Depending on which email client you use, mail headers can be presented with some variation, I’m a numbers guy, you’re a numbers guy…so, we’ll just pick Gmail. Find a message that was ‘promotional’ or ‘notification’ in nature, as this will yield the best demonstration use case. I’ve chosen the fine folks over at Pinterest. If your company already sends mail, those would be fine samples to use as well.

Step 2: Open the email in the reading pane
Adjacent the ‘Reply to button’ hit the down arrow (1), then ‘show original’ (2)

Eh voila, headers!. What you’re actually looking at here is a combination of headers and the message itself, in all of its parts (html, plain text version, to, from, subject, yada yada). For the purpose of this how-to, we’re going to avoid getting overtly technical and focus solely on the header portion. At the bottom of this post, I’ve included links some awesome extra credit reading.

Step 3: Who dis? (e.g. Finding the sender)

Let’s look for ‘who’ sent the message. This isn’t as easy to answer, as you’d think. Reason being is that some of the coolest companies on the planet actually make a living at helping other companies send their email (see what I did there? snort snort). In the spirit of keeping things basic, we’re going to show you a single-party example. You should know however that it’s not uncommon that a single message references multiple responsible parties. In the case of my example, it was transmitted using SparkPost.

Here are the three header values that carry the ‘who’ part.

1. ‘From’: This is the entity that the recipient see’s in their email client.

2. ‘Mail From’: This is the entity that is used for Sender Policy Framework (SPF). The SPF entity uses special DNS entries to ensure that the IP address used to transmit the message is authenticated. This header field has a few other names; Return-Path, envelope-sender and bounce domain.

3. DKIM domain: This is the entity that has signed the message. Signing a message is an important practice to ensure that the message was not altered in transit. DKIM also uses special DNS entries to ensure authenticity.

Step 4: I think he’s Five-oh (e.g. Checking Authentication results)

Now we know who sent it, let’s make sure this message was really sent by Pinterest.

1. SPF: You want to see a string match for ‘spf=pass’

2. DKIM: You want to see a string match for ‘dkim=pass’

3. DMARC: (separate blog post coming on this one. Be sure to bookmark our blog and check back in)

Congrats! You now know about 1000% times more about email then your downstairs neighbor.