Email bounces explained

3 minute read

It could be argued that email delivery is as much art as it is science. Sure, it should be a scientific process because it’s all handled by machines and protocols. However, perhaps for added security but often for less noble or logical reasons not all mail servers play by the rules, so email delivery can be far from predictable.

When your email arrives at your recipient’s server, that machine has four main choices. It could:

  1. Put your email in the recipient’s inbox (hooray!)
  2. Put your email in the recipient’s junk box (boo-hiss)
  3. Inform you that it can’t / won’t deliver the email (sniffle, snob)
  4. Do absolutely nothing (how very dare it!)

Choosing between items 1 and 2 is another dark art which is beyond the scope of this article. Proportionally option 4 is rarely taken, but marketeers should be aware that this does happen, and that sadly we can only group these with the recipients who do receive your email but don’t perform any trackable action with it (load images / click a link).

The point of this article (now I’ve eventually got there!) is how handles instance 3: when a recipient’s email server tells us it won’t / can’t deliver your email.

When this happens, the email server will return a code which should give us an idea as to why the email wasn’t delivered, and we try to split these into two categories: permanent errors (hard bounces) or temporary errors (soft bounces). More information on these codes is available on our help pages.

If we detect a hard bounce we’ll immediately suspend the email address within your account so as not to further aggravate the destination server. Don’t worry though – you’re not charged for suspended addresses, and if the owner of that email address does anything to suggest that they are active once again (e.g. they open a previously delivered email, or visit our subscriber panel) we’ll immediately unsuspended them.

If we detect a soft bounce we are rather more lenient. We’ll look at previous send attempts to that email address, and at your general reputation as a sender. Normally we’ll accept up to 3 soft bounces spanning at least a couple of weeks before we make a suspension decision.

Unfortunately it is common for us to receive codes referring to “undefined status”. In these cases the destination server offers us no further explanation for its rejection. Probably best to look on the bright side; it could have dropped the email and not told us anything at all! In these cases our system bases its soft/hard bounce decision on your general reputation as a sender.