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How Bounces Affect Deliverability

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by: Chris Arrendale (@IBMforMarketing)
20 September 2010

Bounces can be an email marketer’s worst nightmare. Imagine a twisted version of the proverbial bad dream in which you’re trying to run but your feet won’t move, only in the email world you’re trying to send messages but they’re not going anywhere, victims of previous bounces that are now causing email deliverability issues for you at ISPs and inbox providers.

In the real world, ISPs monitor and watch for IP addresses that continually send to bad mailboxes and get blocked due to complaints—and they will adjust your reputation accordingly.

There are two types of bounces:

  • A hard bounce is a permanent email delivery failure. Some reasons for a hard bounce would be sending to a mailbox that doesn’t exist (bad mailbox) or a domain that doesn’t exist (bad domain).
  • A soft bounce is a temporary email delivery failure. Soft bounces can be caused by sending to a mailbox that’s full or having an ISP temporarily block emails being delivered from your IP due to reputation issues.

Some tips for handling hard bounces:

  • Maintain a bounce rate of less than 10 percent, as recommended by most major ISPs. The lower the number of hard bounces you receive, the better your reputation with ISPs.
  • When moving from one ESP to another, make sure you import your suppressions. That way, you won’t resend to the same hard bounces again and ruin your reputation right out of the gate.

And some tips for handling soft bounces:

  • Don’t retry sending to “soft bounce” addresses too soon. Many email marketers will enable soft bounce retries and re-attempt sending to these addresses within a 24-hour period, but the effective rate of soft bounce retries is pretty low. If many of the mailboxes you’re sending to are full, you may want to hold off on sending to those email addresses until your next send a week or two later. The exception? If your IP is temporarily blocked at an ISP, you can retry those bounces a few hours after the original send.
  • Don’t retry sending to “soft bounce” addresses too often. Think of salespeople who continue knocking on your door every day after you’ve told them to go away. After the second or third time, you’re going to be mad and frustrated. Keep this in mind the next time upper management asks you to set up soft bounce retries every hour for a day!

To keep a handle on both hard and soft bounces, I recommend constant list hygiene and regular monitoring of email delivery reports. Sending to recent and active openers and clickers will keep your bounce rates low and your reputation and deliverability high.


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