Understanding Email Metrics (Part 2)

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Last week, we explored a number of email metrics in part one of this two-part series on data-driven email. This week, we’ll cover the rest of the metrics that SendGrid tracks for its users. You might consider these statistics to be a bit less sexy than clicks and opens, but they are certainly just as critical in painting a full picture of your company’s email campaign performance.

Also, a reader of last week’s post asked if we knew of any resources for researching email metrics from a more general and less SendGrid-specific perspective. We’ve provided a few resources in the comments on our “Part 1″ post. If you know of additional resources that may be helpful to other readers, please feel free to share them in the comments of either of these posts.

Now, on to the rest of the story…

Bounces & repeat bounces

A bounce occurs when a sent message is rejected by either the sending or receiving mail server. Common causes for bounced email messages include a misspelled email address, a nonexistent email address, or a full recipient inbox, to name a few.

Hard Bounce

This type of bounce occurs when the receiving server returns a 500 style error, communicating no additional attempts to deliver to that server or email address are needed.  An example reason for a hard bounce is “no mail box for user.”

Soft Bounce

A soft bounce results from an e-mail message that gets as far as the recipient’s mail server, but is bounced back undelivered before it gets to the intended recipient. This type of bounce might occur as a result of a full recipient inbox.

Spam reports & repeat spam reports

This one is easy. If a recipient of your email feels that they received it in error, or simply did not wish to get the email from you, they may click the “report spam” or “junk” button. Spam reports can negatively affect your reputation and deliverability, so it is very important to make sure you send to people who really want your messages. Also, you should always include a clear, easy way to opt out of future messages from your organization, by way of an Unsubscribe link. Finally, a clear and fair initial opt-in process will mitigate potential spam reports down the road. If you’re ever wondering what is “the right thing to do” in the realm of opt-ins & opt-outs, just put yourself in the shoes of the consumer and ask, “what would I expect in this situation?”

Invalid email

An invalid email result occurs when you attempt to send to an email address that is formatted in a manner that does not meet internet email format standards.  Examples include addresses without the “@” sign, addresses that include certain special charaters, or spaces in an address. This response comes from the outgoing server, since an invalid email is impossible to even attempt to send to its [non-existent] destination.


If you receive a block notification, this means that the IP address from which you are sending has been placed on a black list of some sort. ISPs and organizations work from various blacklists, some of which are reputable and valid. Unfortunately for all senders, some blacklists out there are less reputable and, quite frankly, questionable in their intentions. In the event that you are placed on a blacklist, SendGrid will contact the ISP in question on your behalf and submit your IP for removal, once we’ve determined that your email are in fact legitimate and appropriate.


A deferral occurs when an ISP is for some reason not ready to accept email from your IP address. Instead of blocking or bouncing the message, the ISP will defer receiving the message and wait for you to resend. An ISP may do this because it does not recognize the IP from which a message originates; or it could just be that their system is operating in such a way that they cannot accept the email at that specific time. If, upon your resending, the ISP determines that it is ready to trust you as a sender or their system operations are back to normal, the email will be accepted as normal. SendGrid software is intelligent enough to retry delivery of a deferred email on behalf of our customers. If you have built your own email solution, you will want to build this intelligence into your code in order to avoid having to retry deliveries manually.

In certain cases, SendGrid will “Drop” a message to a specific email address in order to protect our users’ sender reputation. SendGrid keeps ‘email lists’ to track bounces, spam reports, and unsubscribes for each of our users. If a user sends a message to an email address that exists on one of these lists within their account, SendGrid will automatically drop the message (i.e., not send to the address). *Note: SendGrid users can always delete entries from these lists if an email address is erroneously placed on one or more of these lists.

Reminder for SendGrid users: all of the metrics we track for you are available from our web interface or our Event API.

That wraps up our overview of email metrics. We’d love to hear your thoughts and encourage you to share your experiences – how do you collect your email metrics, how do you apply them to your decision-making process, which metrics represent your Key Performance Indicators?

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Director, Developer Relations at @SendGrid. Passionate about bringing people together around things they love. I tweet at @TimFalls.

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