Mailboxlayer API Review – Here’s What I Think Of It

Mailboxlayer is an API for validating email addresses. If you’re collecting emails or sending emails in any way, it’s important to be sure that you’re sending emails to real email addresses, as sending emails to non-existent email addresses can raise your spam score and increase the chance that emails you send will end up in your recipients’ spam folder and drastically reduce the chance your email will get seen. That’s where services like Mailboxlayer and its API comes in — it’ll tell you with reasonable certainty whether a given email address is real.

As we do a reasonable bit of cold email outreach here on (mainly outreaching to strangers asking them to contribute to or link to the many community discussions we publish), I was searching for a good and reliable email validator, and Mailboxlayer is one of the one’s I tried. Here’s the email of me first signing up to it (the Basic Plan is $9.99/month, which gets you 5,000 API requests — with more expensive plans the cost per email address validated is a lot cheaper):

Since then I’ve used Mailboxlayer’s API extensively, along with other validators like

Using Mailboxlayer

Mailboxlayer is very easy to use, and its documentation is good. If you’re familiar with calling API’s, you should not have any problem using it. I personally code in Python and before I email anyone these days, I will run it through this function that calls the Mailboxlayer API (obviously I can’t reveal my API key):

As you can see, I am passing an email address to the Mailboxlayer API and then looking at the following information that is returned:

  • The score. This is basically how certain Mailboxlayer is that the email address is valid and an email sent to it will not bounce. For good emails, you’ll expect the score to be 0.8 or above, so you can see that I’ve had it return True for the email being valid only if score >= 0.8 (if other requirements are also met).
  • The smtp_check. Checks if SMTP (Simple Mail Transfer Protocol) exists on the recipient server.
  • The format_valid. Checks if the email address itself is a valid format.

Mailboxlayer also returns a variable called mx_found (indicating whether there are MX records of the recipient domain, which are used to send and receive email), though I haven’t checked that in the above function.

As you’d expect, you can call Mailboxlayer’s API in any programming language, and there’s nothing difficult or convoluted about it at all. This is a big advantage over another email validator I tried, VoilaNorbert, which requires you to set up a webhook on a server that the API will post to (see here). So Maliboxlayer passes with flying colors as far as its ease-of-use.

The Accuracy Of Mailboxlayer

Like every other email validator, Mailboxlayer will never be able to say with 100% certainty that an email is valid, which is why it returns a score from 0 to 1 indicating its certainty. To my surprise, validating emails is in fact not super simple — it’s not something you can know for sure with just a few lines of code, which is where there’s a need for validator API’s like this that have spent a lot of time building something that can validate emails with reasonable accuracy. As mentioned above, on Mailboxlayer, you can expect the score it returns to be 0.8 or above for good emails. I did a fair amount of testing on Mailboxlayer for emails that are definitely valid, and found that in many cases, it still only returned a score of 0.8, while occasionally for some bad emails it would also return a score of 0.8. So it’s not perfect.

But here’s the bottom line: Based on my experience, when Mailboxlayer returns a score of 0.8 or higher, the email will be valid roughly 97-98% of the time (of course, your mileage may vary). I’ve read you should try to keep your bounce rate significantly below 5%, so if you use Mailboxlayer to verify email addresses and only email email addresses that have a score of over 0.8, you should have a small and acceptable bounce rate that won’t get you into any problems. If you do not use any email validator like Mailboxlayer, you can expect your bounce rate to be significantly above 5% if you’re just emailing addresses that you find on various random websites that you’re trying to get in touch with. There are in fact a lot of email addresses listed online that will bounce or return an error when you email them.

Here are some of my emailing stats when I verify emails with Mailboxlayer:

That’s a bounce rate of significantly below 5% thanks to Mailboxlayer. There are still bounces where Mailboxlayer returned a high score for emails that bounced or had an error, but it gets it right the vast majority of the time.

The Verdict On Mailboxlayer

Based on my experience with it, I can recommend Mailboxlayer. Its pricing is extremely reasonable (similar or better than any other email validator API), it’s easy to use, and it’s accurate enough for it to be well worth the cost.

If this summary of Mailboxlayer has been useful to you and you’d like to try it, you can sign up through this link.

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