• Best Practices

Diving Head-First Into The Inbox: What’s Impacting My Deliverability?

Nick Schafer
5 min read

It’s no secret that many first-time senders struggle with delivery across different mailbox providers. To find out what could be impacting deliverability for new senders, we set off on a journey to uncover as much information as we could about what makes MTAs tick.

What we knew at the start of our journey, was that sending results were not the same across the board. Senders struggling with delivery and engagement with one mailbox provider could very well see fantastic results with another. Check out the sending trends over a 30-day period of our friends at some personal finance company:

These guys were crushing it on Gmail and Yahoo with close to 100% delivery, but barely hitting 50% on Outlook. #sad

What we need to consider is that this is an incomplete picture of what’s impacting delivery rates. There could be other factors not represented here, like where the contacts are in their journey as a customer. Were these contacts being messaged for the first time? Did the emails sent follow best practices to avoid being flagged as spam?

New Contacts

Mailbox providers care about the relationship, you the sender, have with the recipient of your emails. The more your contacts open messages from you, the more you will form a pattern of (good) recurring behavior. Think about it: reputable senders generally have a loyal customer base that they send to frequently. If this is the first time a certain segment of your contacts is hearing from you…don’t expect to be let in right away.

At the other end of the spectrum, you’ve got spammers that generally acquire lists in nefarious ways and blast out messages from new IPs and/or domains and then vanish. Since sending to a massive list of new contacts mimics this behavior, when mailbox providers see a high rate of first-time messages from a single IP, they may get a little trigger happy and block the IP. At the end of the day, mailbox providers look out for their customers to keep them safe from spammers-which is why they track these sorts of metrics— and I can respect that.

What does this mean for you? It’s common practice to separate your transactional messages from marketing messages (still a great idea), but if you are only sending things like welcome emails to recipients you’ve never had a relationship with, make sure you are also mixing in other types of transactional messages like confirmations; password resets; etc. This will help to even out those messages sent to first time recipients.

Complaint Rate

So, what else did we find on our journey to explain deliverability hurdles? Two words: junk mail.

Unwanted mail can now generate complaints when messages hit the junk folder at different mailbox providers. Every time your contacts mark your messages as “Junk”, it’s factored into your overall complaint rate, which considers the number of messages marked as junk, over the number of emails sent in a specific email program. Best practices tell us that this complaint rate should be less than 0.1% (one-tenth of one percent) with each ISP, so if your complaint rate is much higher than this, consider going back to your list and check for contacts with low engagement for future email sends.

Keeping track of your complaint rate as a performance indicator provides great insights into user behavior – and Mailgun captures these complaints, too- which means you’re in a better position to offset junk mail complaints. You can understand which email campaigns continue to add value for recipients, and which ones you can nix. So, don’t fret if you see a few more complaints. Yes, be mindful and make adjustments as needed in your email streams but count on Mailgun to help you proactively monitor your sending performance.

Retroactive Junking

Last but not least in our findings, is that your sending reputation-past and present-matters at all times. Did you know that your message can be delivered to an inbox, only to later be relegated to the junk folder? This happens with several ISPs when the reputation linked to a message changes after it was delivered.

Think of it like this: spam filters get things wrong sometimes (they’re just like us, right?), but if general sentiment towards messages from a sender is poor, then that message that got inboxed may be moved to the junk folder. It goes back to our initial understanding of relationship monitoring: the goal of ISPs is to protect their end users from spam, so they are going to look at every piece of information they can to do just that. You don’t want a throwback to the 90’s when your inbox looked like this:

In short, send messages that people genuinely want to receive and this won’t happen. You may be able to trick systems temporarily, but they will catch up to you in the end by not following email best practices.


You’re not alone if you’ve been experiencing issues with delivery across different mailbox providers. With the findings we’ve shared here, we hope that you can get things back on track, and at the very least have more clarity around what’s happening to your messages when they reach the inbox. If this all sounds like too much to handle, talk to us about our Managed Service. You could have a dedicated technical account manager to proactively monitor your account, and help you achieve better deliverability.


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Last updated on August 28, 2020

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