When emails are delivered but rejected later

Mail servers can operate in different ways and depending on the setup it can lead to confusing results in relation to delivery stats.

You may have heard the term “Returned Mail”, which is interchangeable with “Asynchronous Bounce“, and it’s these emails that come back after pressing send that means “Delivered” doesn’t always actually mean “Delivered”!

If you’ve ever encountered this scenario then you’ll know it’s a bit of a head-scratcher so read on if you want to find out what is going on.

3 types of email delivery failure

When email delivery results in failure it will be down to one of these three reasons:

  • Immediate rejection by the mail server
  • Timeout / communication error
  • Asynchronous bounce

Immediate rejection

When an email gets sent it kicks-off an SMTP conversation with the receiving mail server and at that point the email might get rejected as a hard bounce such as “bad mailbox” or soft bounce such as “mailbox full”.


Sending and receiving mail servers need to be able to talk to each other and if the conversation cannot be initiated or is taking too long to conclude then the sending mail server will terminate resulting in a soft bounced email.

Asynchronous bounce

This is the cheeky one that can leave you puzzled because the receiving mail server may say “thanks” as if it will be delivered but at some point between accepting the message and landing in the inbox it changes its mind and wants to reject it.

When this happens an email is sent back to where the email came from to say “Oi, I don’t want this!” i.e. a Returned Mail.

Why does it happen?

In relation to the accepting mail and then rejecting it later that’s a completely valid setup and has been used for years by many different providers.

The things that can happen in-between accepting and then rejecting are things like…

  • Content scanning & filtering (checking for spam)
  • Storage space (is the mailbox full)
  • Reputation (is the sender a spammer)
  • Local policies (recipient has local block / rejection policies)

…and there could be lots of other things happening as well. Either way, the sender has no control over this so it’s something to be aware of.

What are the effects and is it a problem?

Returned Mail itself is not a problem but it is a good indicator that there are problems with your email campaigns.

Pretty much every marketer will get the odd Returned Mail, it’s when the amount of Returned Mail increases you should be investigating why.

The effects of Returned Mail vary and here’s a few of the most annoying ones.

False opens & clicks

When email content and links are scanned by mail filter software it can result in opens & clicks being recorded – if the email then gets rejected it not only throws the campaign stats out but also means retargeting and follow-ups aren’t going to be reliable.

Reputational damage

When you think emails are actually being delivered you won’t be suppressing them which means every time you send to these email address the receiving mail server will become increasingly annoyed seeing as though it keeps sending the messages back saying “No thanks”.

Not always “bad mailbox”

If all Returned Mail was related to mailboxes that do no exist they could easily be removed from your list, but that’s not always the case. It complicates the management of Returned Mail so then it’s down to you to decide how you deal with it.

Can anything be done to help?

Well that depends on your ESP but from an Instiller point of view there’s a few ways in which we deal with Returned Mail.

Data download

All Returned Email is collected and made available as a download in the associated campaign report. The download provides information from the email to give an indication as to the reason why it was rejected e.g. bad mailbox.

The data in the download can then be used to clean up your list which will reduce the amount of Returned Mail in future campaigns.


We have a customisable set of auto-cleansing rules where thresholds can be set to suppress email addresses automatically. A simple example being where we see a bad mailbox it’s one bounce and it’s out.

There’s a rule for Returned Mail, which by default has a higher threshold of 10, and means over-time email addresses that continually lead to rejection after delivery will eventually be suppressed.

Parsing email content

This is something new we’re working on and the idea is to identify what type of Returned Mail each message is so that cleansing can be more intelligently applied to…

  • Bad mailbox
  • Mailbox full
  • Out of Office
  • Reputation related

Need some help?

If you’re familiar with some of the headaches above then why not take a free and unlimited trial to see how the features of Instiller can help to relieve some of the pain.

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