The Hitchhiker’s Guide to Mailbox ProvidersIf you’re a nerd of a certain age (as I surely am), there’s a good chance you have a special place in your heart for the trials and tribulations of an earthling named Arthur Dent, his optimistic friend Trillian, and their adventures exploring the galaxy in the company an eccentric travel writer for The Hitchhiker’s Guide to the Galaxy. Oddities abound: The bureaucratic Vogons. Marvin the paranoid android. The babel fish. But, above all the absurdity and puns, there’s a clear reassurance: don’t panic!

I’m here to deliver the same message to anyone struggling to navigate the universe of mailbox providers. There’s a good reason why webmail and internet service providers (ISPs) require email senders to follow various rules and protocols if they want to reach the inbox: to help ensure their customers have a great experience, free of spam. But the complexity of the inbox universe can be challenging if you’re new to the email space, whether you’re a developer building email into your app or business process or a growth marketer using email to nurture customer engagement with your service. You’re not alone if you’ve wished for something like a babel fish to make sense of the idiosyncrasies of the email industry.

But like I said, don’t panic! Even though mailbox providers—from smaller ISPs to the biggest like Gmail—keep some proprietary details close to their vest, email sending best practices aren’t as mysterious as it might first appear.

Here are the basics any intrepid email explorer needs to navigate the galaxy of mailbox providers:

  • The universe of email inboxes is dominated by a few big mailbox providers, especially for consumers. For a decade or more, it’s been “The Big 4”: Gmail, Outlook/Hotmail, Yahoo!, and AOL, with a handful of ISPs like Comcast rounding out the landscape. (As more businesses move their apps to the cloud, business inboxes are beginning to behave more like consumer inboxes.)
  • It’s true that the path to the inbox has gotten more complicated, thanks to bad actors like spammers. But if you understand what happens after you hit send, it’s easier to solve deliverability problems when they do arise.
  • Nearly every major ISP has published postmaster resources specifically aimed at helping senders do the right thing. You can find tools to look up bounce codes or to help validate your DKIM, SPF, and DMARC implementations.
  • ISPs and mailbox providers really do want you to understand what you need to do to be successful. Listening to your customers’ signals—with the help of a babel fish, if necessary—is where it all begins.
  • Even with all the information ISPs share, sometimes a guide is the best way to help you find your way to the email inbox. Whether it’s The Developer’s Email Survival Guide or 17 Ways to Improve Your Email Deliverability, we’ve got your back. And it’s why SparkPost’s expert deliverability and other technical services are so important to our enterprise customers.

It’s true that the universe of email mailbox providers has its unexpected quirks and legitimate challenges, and we may not ever truly understand the answer to the ultimate question of life, the universe, and everything. But armed with the right knowledge and the support of a great team, the restaurant at the end of the universe is in sight.


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deliverability_panel_eecI’m in Miami wrapping up #EEC15, The Email Evolution Conference. The closing Keynote Panel Discussion organized by Dennis Dayman and Ryan Phelan was worth the price of admission; the panel they organized featured key postmaster/abuse personal from Comcast, Hotmail/Outlook, AOL and Gmail. Although regular fixtures at M3AAWG meetings, the folks in this room don’t normally have a chance to hear first hand how an ISP measures activity within the inbox and responds to emails as they arrive. There were furrowed brows, heads nodding and a plethora of other emotions as questions were asked, assertions made, accusations levied and laughter enjoyed by all.


One of the most important things I took away from this session and I think you may find this valuable too are the signals that most ISPs read as good vs. bad. Here’s a cheat sheet and take away that you can help you better understand how engagement, which was called ‘a philosophical principle rather than a secret sauce’, is measured by a mailbox provider or an ISP. One of the major disconnects I’ve seen between senders and receivers is the idea that engagement is a single measure. Quite the contrary, senders are not privy to the metrics that receivers are and vice versa, there may be some attribution, analysis through web behavioral data, that leads to a very different picture of engagement on the sender side. This is the fundamental conundrum that I think is best represented by the metric system vs. the english system. Both are valid (well one’s more practical than the other), both are capable of measuring the same distance but use different units. One doesn’t invalidate the other, hence the philosophical nature of the construct.

Positive Signals

  • Open an email
  • Adding a sender to the address book
  • Moving a message to a specific folder (filing)
  • Rescuing a message from the spam folder
  • Replying to a message.

Negative Signals

  • Deleting a message without opening it
  • Marking a message as spam
  • Reporting phishing

Thankfully there are more positive signals that inform engagement at a mailbox provider than negative ones. One thing to consider: I’ve seen a number of senders instruct their recipients not to reply to an email, that the email box will not be checked or monitored. Given that replying to emails is a positive signal that will ultimately improve a sender’s engagement, leading to better reputation and finally deliverability, it might be worthwhile to make the sending addresses accept replies and even review them for customer correspondence.