We all get email newsletters.

When we talk about advertising emails, we are actually talking about the digitally transmitted content such as news, offers, promotions or other information. Very few end customers care about the technical side of things – as long as the subscribed advertising email is in their inbox!

But when this is not the case, then they want the email they were expecting. The user contacts their email provider and complains. And then it happens: The mailbox provider tells them that the sender’s emails were not properly posted and, therefore, cannot be delivered to the mailbox.

Hmm, sound familiar? Perhaps you can see some parallels to the last conversation with the post office when your letter was returned unsent? Or to the feedback from the service employee at the postal service’s call centre? It is indeed comparable.

Let’s start by clarifying what I mean

So, what are we basically talking about here? That’s right, we’re talking about electronic direct mail, and as old-fashioned as that sounds right now, that is exactly what it is. Things have not changed for decades (yes, you read that right, it’s been four decades since 1981).

The transmission of information as a digital letter from the sender to the recipient takes place via a defined delivery route, and, just like a visit from a Head of State or monarch, according to a fixed protocol – the SMTP = Simple Mail Transfer Protocol. (For those who really need the details, I recommend the RFC5321 guideline as bedtime reading).

But that is where it starts! Who is defined as the sender? And who or what is the recipient? What do all the technical terms and standards mean, and why is it not as simple as a postal letter? Why does digital technology always have to be so complicated?

Don’t worry; it’s only half as complicated as it looks. It has been explained to me many times and I was not born knowing this. To define it in a comprehensible way, we can use the analogy of the classic postal letter.

And to lower blood pressure – professional email service providers know exactly how to meet these requirements. Nevertheless, every email marketer should at least roughly understand how it all works.

The sender

The sender is actually the senders, as in most cases there are two of them. We can call them the author (sending party) and the sender (email service provider), then the context and, above all, the responsibility becomes clearer.

Roughly speaking, the author is responsible for the visible appearance of an email, so for how it is displayed in the mailbox, and for the content of the email. The author has the actual relationship with the addressee, consequently the content and the selection of the appropriate addressee by the author also triggers delight or frustration.

What does that look like? With delight, there’s the urge to buy, the identification with the author, with the brand, with the product, with the service. Frustration results in ignorance at best, a spam complaint or an entry on a block list at worst.
Note: The author maintains the customer relationship.

The author’s responsibilities and accountability:

1. The correct time of dispatch – Date
2. Meaningful sender address – From
3. Reply possibility to the correct address – Reply-to
4. The correct addresses of the addressees – To
5. The Subject Line
6. The right content – Body (we’ll leave that out for now)

In the email header, this part then looks like this:

When (Date:) the author (From:), to whom I can always reply directly, by the way (Reply-to:), sends a message to the recipient (To:) on a certain topic (Subject:).

This summarises the visible components of an email header.

In our case, we understand the sender to be the email service provider. This is the party who puts the content in the envelope and makes sure that the content does not get lost. They write the address on the envelope and stamp it correctly. Actually, it is the prudent assistant who carefully packs the elaborately written content and ensures its dispatch. The sender hands the letter over to the postal service. However, in order to do this correctly, the email service provider needs a clean foundation from the author. The correct addresses, as well as a correct and meaningful return address.
Note: The sender can only do a good job with the means they are given.

The sender, therefore, acts on behalf of the author and thus bears the following duties and responsibilities:

1. The shipping address used on the message envelope – Mail-From address (this is not directly visible in the email header – spoiler: Return Path – but we’ll get to that in a moment)
2. Seal and lock the envelope so no one can tamper with the message or interfere with the transmission – DKIM signature (the professionals among you will now say: “And the TLS encryption!” – yes, you’re right, but that’s going too far here).
3. Put the author details in the correct format – all of the author’s headers – Date, From, Reply-to, To, Subject.

With all this information, the sender gets the message ready to be sent:

Consequently, for a message to be delivered successfully, the writer and the email service provider must both do a well-considered job.

The recipient

Let’s now turn to the sender’s counterpart , or rather counterparts.

Again, we are talking about two parties; a team that has to work together. To stick with the analogy, I would call these two the carrier (mailbox provider) and the addressee (email user). As described before, the relationships here are between author and addressee and sender and carrier.

The sender hands the envelope with the message inside to the carrier and now the carrier fulfils their task and does their part of the job.

Is the envelope intact and undamaged or is there visible damage that is already visible at the time of delivery? Of course, the carrier does not want to be liable for these afterwards, because they also want to offer their addressee a good service. There are the same interests here as there are between the sender and the author.

The carrier can only do their job if the dispatch details and the correct franking are available. Only complete addresses and correct franking lead to successful delivery to the addressee, otherwise the message goes to the sender by return of post. The carrier thus ensures that the message reaches the correct addressee if all the conditions for delivery are met.

The addressee then receives the message from the carrier in their inbox. The messages pile up, because the addressee often only looks at their inbox once a day, maybe even less often. If the addressee then does check their email, then, emails are sorted and sieved out. The known and intriguing emails are read first. The addressee does not read unknown and questionable material or just deletes these directly. So if the author wants the addressee to read their message, both should know each other well and they should write something important and interesting. The visible sender address and subject line are, therefore, the key to success.

On the recipient’s side, the advantage of an email over the classic letter is clearly visible. This is because the carrier often logs their activity during transmission and can thus create transparency for the knowing layperson as to whether everything went correctly during transmission.

And how do they do that exactly? Very simple – they write it in the header of the message – What a service, brilliant!

1. They noted the sender address (Mail From) in order to send the message back to the sender in case of discrepancies – Return Path:
2. The time of delivery of the message from the sender to the carrier was also documented, and not only that, but also who the sender was – Received:
3. The franking was correct, the sender officially had the author’s order and permission to handle the message on their behalf – Received-SPF:
4. The carrier also noted whether the letter was intact when it was handed over – Authentication-Results:

• • These, in turn, contain who received and checked the message – mta4711.mail.box.provider.tld
  • That the envelope is externally intact – dkim=pass (ok) header.i=@sender.tld header.s=selector01;
  • That the contained message is also intact in the intact envelope – dkim=pass (ok) header.i=@child.brand.tld header.s=selector02;
  • That the franking was checked and correct – spf=pass smtp.mailfrom=@child.brand.tld;
  • In case of security concerns or third party interference, the sender had briefed the instructions for further action to the carrier – dmarc=pass(p=reject sp=NULL dis=none) header.from=child.brand.tld;

I know it got a bit complicated there, but good service is not easy and always a bit more demanding.

In short, when the message is delivered to the addressee’s mailbox, both the message itself and the documentation of the delivery process are documented and delivered. In the email header, it looks like this:

The message and the addressee

After the delivery is before the delivery. This means the user’s reaction to the email after it has been delivered to the inbox influences the delivery of future messages.

At worst, a spam complaint to the mailbox provider. The aim, however, is to elicit positive responses – reading, clicking or forwarding the message. This has a positive influence on the reputation.

But there are other options that allow the mailbox provider to avoid direct spam complaints, for example. Additional mandatory email headers are used for this purpose.

For example, instead of a spam complaint, many mailbox providers offer unsubscribing from the email list using a List Unsubscribe header. Or if there are spam complaints, then the sender at least receives feedback about this and can then clean up their data – this is where so-called feedback loops come into play.

If a savvy user does not want to use any of the above procedures, Certified Senders also have the option of filing an official complaint with the Complaints Office of eco – Association of the Internet Industry. For this purpose, each certified sender is required to include another additional header – the X-CSA Complaints Header.

These additional headers for processing user feedback are, in summary, the following:

1. For the additional option to unsubscribe from email lists, the mailbox provider needs the where and the how at least in the form of a URL – List-Unsubscribe: and List-Unsubscribe-Post:
2. To use the feedback loops, email must also contain special IDs / headers in some cases – Gmail in particular expects a Feedback ID, other MBPs use the Message ID as email header – Feedback ID: and Message ID:
3. For a complaint to the Complaints Office, the X-CSA Complaints header must be inserted – X-CSA Complaints:

In the email header, it looks like this:

At this point, the email differs a little from the classic letter, which is not a bad thing and, in this case, it helps the author and the sender to check their strategy and to keep the data clean.

Summary

In a nutshell: The carrier receives the written letter from the sender in order to deliver it to the addressee after a thorough examination.

This single sentence describes the entire interaction of all parties involved in the digital automatic mass mailing of email and their delivery to the inbox. Since several parties are involved in the process, the wheels have to interlock accordingly, and everyone has to do the right job in their own field.

So the author cannot hide behind the sender if they don’t fulfil their responsibility. Just as the sender cannot hold the carrier or the author responsible if they do not fulfil their obligations. Carriers and addressees also have an influence on a smooth process because only feedback helps to uncover errors and avoid them in the future.

The complete email header from the message in the inbox

So up to this point, we have dissected and explained the email header piece by piece. In the email message, the whole thing then looks like this: