Postmark Express - Journey to the inbox


​​All these fancy email words flyin’ around your head like a cow in a tornado? MDA, MTA, MUA, OMG, am I right? Well, we roped up some definitions for you that will hopefully make things a little more easy to lasso your brain around.

MTA Mail Transfer Agent

A software that transfers emails between the computers of a sender and a recipient through the powers of the SMTP and the INTERNET!

The MTA receives the email from the MSA and determines how to transfer the email to the recipient by searching through name servers and the DNS for their location. If the recipient is not on the same server (for example, a Gmail recipient emailing and Outlook recipient) then the MTA relays the message over to the recipient’s mail server where it is then handled by the recipient’s own MTA. There may even be additional relays in-between the sender’s mail server and the recipient’s.

In Journey to the Inbox MTAs are represented as local post offices, managed by a teller or in Outlook Oasis’ case, the world famous Wild Ear Hickok. They take outgoing messages and are able to look up the recipients whereabouts for delivery.

msa Mail Submission Agent

A server program which is sent the outgoing email from an MUA, checks the email for any issues or errors, and then transfers it via SMTP to the same server’s MTA. There’s a lot more to it, with lots o’ fancy words like ESMTP and ports and stuff, but let’s just keep it simple, shall we?

Stagecoach Mary represents the MSA in our tale, guiding the email from the MUA (Gmail) to the local postoffice which acts as the MTA. Typically the MSA’s job goes no further than the MTA, but luckily Jordan caught Mary on her lunch break and she accompanies him for the remainder of the journey. What a thoughtful pup, she is!

mda Mail Delivery Agent

Another server program, this time on the recipient’s mail server. The MDA (also affectionately known as the Local Delivery Agent) receives the email from the server’s MTA and hightails it over to the recipient’s mailbox.

Good ol’ Calamity Jane plays the role of Outlook Oasis’ MDA in Journey to the Inbox. Just as Stagecoach Mary (MSA) helped bring the email from the sender’s MUA to the sender’s MTA, Calamity Jane helps Jordan bring the email from the recipient’s MTA to the recipient’s MUA. Don’t let the cone of shame fool ya! Calamity Jane is the most legendary MSA this side of the Rockies!

mua Mail User Agent

Also known as an email client, the Mail User Agent is a computer application that allows receiving and sending emails. They can be webmail email services like Gmail,, Yahoo, etc, or they can be software applications like Thunderbird or Lotus Notes. Basically, what the human recipient user interacts with to read and respond to your email.

In our lil’ ol’ story, Jordan sends his email using Gmail, which is the sender’s MUA. The email is then delivered to Lucy’s mailbox, which is the recipient’s MUA.

MUAs create and send email messages, passing them on to the MSA which then transfers it along to the Mail Transfer Agent (MTA).

dkim DomainKeys Identified Mail

DKIM (DomainKeys Identified Mail)is an email security standard designed to make sure messages aren’t altered in transit between the sending and recipient servers. It uses public-key cryptography to sign email with a private key as it leaves a sending server.

Recipient servers then use a public key published to a domain’s DNS to verify the source of the message and that the body of the message hasn’t changed during transit. Once the signature is verified with the public key by the recipient server, the message passes DKIM and is considered authentic. This check can happen during any and every part of the delivery process, from various MTAs to the MUA.

However, just because DKIM passes doesn’t mean the message is above suspicion. The DKIM-Signature also assigns a “responsible party” to the message. For example, Postmark uses its own domain to sign DKIM for every outgoing message, and receivers weigh Postmark’s reputation *heavily* when filtering mail.

ETF has a great way of putting this:

“DKIM allows an organization to take responsibility for transmitting a message, in a way that can be verified by a recipient. The organization can be the author's, the originating sending site, an intermediary, or one of their agents. A message can contain multiple signatures, from the same or different organizations involved with the message. ... This permits verification of a responsible organization, as well as the integrity of the message contents.”

In the comic, DKIM is represented by the sealed and signed envelope of Jordan’s message. Wild Ear Hickok, the receiving MTA, sees that the message is sealed and signed by a verified, trusted sender. If only setting up DKIM was as easy as licking an envelope!

dns Domain Name Service

DNS (Domain Name Service) is kind of like the phonebook (or your phone’s contact list for you young’uns!) of the internet. Basically a directory that translates a domain to corresponding IP addresses so that your browser can load the proper resources.

How DNS works is a fairly complex process, so why not let take a look at dnsimple’s awesome webcomic about how DNS works! Who knew comics could be so educational, eh?!

In Journey to the Inbox the recipient MTA teller looks up the email recipient’s location by contacting DNS who in turn was able to look up the recipient’s location and relay that information back to the MTA for delivery instructions.

spf Sender Policy Framework

Sender Policy Framework (SPF) is an email authentication method that helps prevent them pesky spammers from sending emails on behalf of your domain. Nice try, spammers!

An organization can publish to DNS a list of mail servers and IP address authorized to send messages using their domain. That way, the email recipient’s mail server is able to to tell if the sender was authorized to send the message or not by checking the sending IP address.

SPF is kind of a weakling by itself, but when combined with DMARC it can help detect spoofing!

Jordan’s email in Journey to the Inbox is SPF authenticated using an SPF stamp. The analogy is a bit of a stretch, but it’s basically telling the recipient MTA that “this message is allowed to be sent by this specific delivery dog.”

nsec Nanosecond

A nanosecond (nsec or ns) is one billionth of a second.

When Mary suggests they take a break for a nsec, it might not seem like a very long break but when the whole email delivery process can take only a few seconds a nsec isn’t so fast. Well… it is, but come on. Joke’s gotta joke!

helo 👋

HELO is a command used for SMTP transactions that helps start the process of an email transaction. Basically, a way of saying “What’s up, I’m Let’s do this thang!”.

This can occur between MTAs, and often with MDAs and MSAs, and sometimes when communicating with MUAs (though they might be using something like POP or IMAP instead of SMTP).

EHLO can also be used and supports more SMTP add-ons. But having our teller say “EHLO” wouldn’t make much sense. Unless… she was from Australia, maybe? “EHLO mate!”

IP Reputation

The internet is essentially billions of machines/devices sending data between each other using various “internet protocols” (IP), which are the rules and standards for how data is formatted during transfer. An “IP address” is an identifier assigned to one of those machines.

Over time, that IP address develops a reputation, much like the one we build as people. It’s based on what you’ve done in the past and who you’re associated with, and that reputation can vary based on how much that person/organization knows about you.

No one wants to accept data from IP addresses known to send spam or cause complaints. So when it comes to email, you’ll have better delivery when the sending IP address has a good reputation with each receiver you send to.

As you might know, there are a heck of a lot of factors for email delivery, and IP reputation is just one piece to the overall puzzle. Postmark wrote up a few details on IP and domain reputation, their differences, and how they can impact one another here!


Email authentication are techniques used to provide verification of a message’s origin and sending domain ownership to help prove the message’s legitimacy and that it was not forged by spammers and phishers.

The three most widely adopted authentication methods are SPF, DKIM, and DMARC.

An email’s authentication then helps a receiving email server determine what actions to take with the incoming email. Proper authentication helps reduce the chances of the email getting caught in spam filters and improves chances of reaching the inbox.

Dogecoin 🐶🪙

As more and more cryptocurrencies came into existence and their legitimacy questioned, two software engineers decided to make their own payment system as a joke and soon after Dogecoin barked its way into existence. Over the years, Dogecoin’s popularity and legitimacy has only increased, becoming one of the top cryptocurrencies out there. Use it to purchase web hosting or even buy donuts.

Featuring the beloved Shiba Inu dog of the "Doge" meme fame, Dogecoin’s jam is all about providing a fun internet currency in a way that sets itself apart from the rest of the dog pack.

If the Dogecoin references in the comic didn’t make you roll your eyes, then you’re just not hip. Woof cha-ching!

Email Reply Train 🚂📧📧📧

An email reply train, or just email train, refers to a long thread of emails that occurs when the email’s recipients reply without deleting the previous replies, or when a recipient replies all instead of responding to the original sender individually.

You know what we’re talking about. Being stuck in a work email thread that grows more unruly by the minute as more coworkers click the dreaded “Reply All” button to share their thoughts. Soon you’re not even certain what the original email was about. Oh goodness, I’m getting anxiety just thinking about it!

Email trains can be useful as a means to track a conversation between a group of recipients, but it’s always a good idea to ask yourself “Does everyone on this email need to see my reply?”

The answer is almost always... no.

And the dress is clearly blue.

Spam Filter

Software implemented at any/all levels of email delivery to help with the detection and prevention of unwanted mail. They typically use a combination of content scanning and referencing block/allow lists (public or private).

The phrase “spam filter” could include firewalls, routing mail through a separate email server (third party security service), software installed directly on the recipient’s mail server, or even filters set up by the individual recipient within their own inbox.

Spam Filter Spot is Outlook Oasis’ spam filter, and she’s no joke! While the spam filter might actually come into play before the email reaches the MDA (instead of after like in our story) the point to take away is that the spam filter one of the last big hurdles an email encounters when making the perilous journey to the inbox.

Spoofing 👻

A tricksy technique used in spamming and phishing attacks. The spoofer tricks an email’s recipient into thinking the email was sent from a person they trust, like a friend, coworker, or legitimate business. In reality, the email was sent from a bad actor (no, not Nicolas Cage but someone who intentionally engages in actions that are harmful, illegal, or just morally wrong…so maybe Nic Cage?!).

There are lots of different ways bad actors can try to “spoof” an email. They could send from a domain that doesn’t have authentication methods in place to verify legitimate mail (SPF, DMARC, etc.), they could “hack”/compromise a machine already in the network they want to send from, or they could even send from a “lookalike” domain that visually tricks the recipient into replying back with some sensitive information like login credentials or credit card details. Authentication tries to combat some of these tactics, and spam filters and firewalls try to combat some of the others. Still, stay on your guard!

In Journey to the Inbox, ol’ Nosey McShnoz attempts to spoof Jordan by delivering a spam email under his name. He dresses the part, and even says his name is Jordan, but luckily Spam Filter Spot is trained to sniff out such spam attempts and was able to thwart the attack! Not all spam filters are as keen-nosed, though, so always be on the lookout!

Fun fact! DMARC Digests gives you visibility into who’s sending emails from your domains so you can quickly identify possible spoofing and phishing attacks!