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Email Deliverability 101

by Colin Nederkoorn

Writing great email copy ain’t worth jack if your emails don’t make it to the inbox. Here’s a quick run down of the critical things you need to improve and track your email deliverability.

I learned a lot from reading boring technical articles online and from speaking to Ev Kontsevoy of Mailgun who spent an hour on Skype chat last year with John and me talking about deliverability. Thanks Ev!

First up: Getting rid of “via” on your emails.

You might notice a bunch of emails in your inbox say “via”. This basically means the sender hasn’t properly authorized someone else to send their mail. Here’s what that looks like in Gmail:

Setting up SPF and DKIM

There are two pieces to get rid of the “via”. We recommend people do both. Those are SPF and DKIM.

1. SPF (authorizing senders)

A lot of companies don’t run their own mail servers and use a third party. In order to tell the world that “Sendgrid” or another ESP has the right to send on your behalf, you can add a Sender Policy Framework (SPF) record to your DNS.

Recipient ISPs can verify that an email was received from a computer that you’ve approved to send on your behalf.

2. DKIM (digitally signing your emails)

DomainKeys Identified Mail (DKIM) is a way for emails to be signed so that a recipient can verify that it’s truly coming from you. You add a record to your DNS and email received from you is checked against that record.

Warming up a dedicated IP Address

Your mail reputation is like a credit score. No history is the worst thing you can have.

Like a credit card, there are also some very fuzzy “limits” on how much mail you can send without tripping alarms.

I was speaking with someone who runs their own mail servers. Usually they send 1000 emails a day. Once every month or two they send 200,000 emails all at once.

They have problems with deliverability. Why?

Internet Service Providers (ISPs) don’t receive big volumes from them on a regular basis and have fail-safes to prevent spammers' and scammers' mail getting through. ISPs assume that their giant mailing means their mail server has been compromised and block their mail automatically.

If you have your own IP address, you want your mail volumes and rate of send to be consistent. You’ll need to ramp up sending volumes over time or risk being blocked.

For irregular sending volumes, you’re probably better using a pool of IPs shared by others where aggregate sending volumes are consistently high.

Most newsletter providers use a pool of IP addresses for sending your mail.

Sender Score

A quick way to sanity check how ISPs might view your sending reputation on a particular IP address is with Sender Score.

Here’s ours:

Customerio Sender Score

We are generally around 98 - 99. Return Path has some additional services they sell around this, but I don’t know anything about them. Do any of you pay Return Path? I’d love to learn more about the benefits your company gets from it.

What to do if you get blocked

Even if you do all the right things, you can get blocked.

There’s an inner circle in email. The big receiving domains in the US – AOL, Comcast, Yahoo, Google all have people who can unblock you.

If you work with an ESP, most larger ESPs like Sendgrid have someone on the team with relationships with the people at those companies. So you call Sendgrid, Sendgrid calls Comcast, and you’re unblocked.

Otherwise, you gotta wait it out, and hope for the best.


These are some quick fixes that get more of your emails into the inbox and don’t get automatically sent to spam. Maybe 20% of people who sign up for our product know about SPF and DKIM so you may now be more ahead of the deliverability game than you think.

Have other tips and tricks to share or questions about deliverability? Tweet us @customerio or send us an email!

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