Why Do So Many Emails Go Undelivered?

Before you can optimize email delivery, you must understand the common causes of emails not reaching their destination. Here are some factors that prevent an email's delivery.

ITPro Today

December 22, 2014

2 Min Read
Why Do So Many Emails Go Undelivered?

Believe it or not, delivering emails isn't as easy as simply clicking the "send" button. In fact, according to research from Return Path, more than 20 percent of emails actually go undelivered. This staggering number is an alarming concern for anyone who is reliant on email performance—both senders and receivers. In fact, the issue of email deliverability has been a problem for at least a decade, and it will likely continue to remain a concern for some time.

If your email strategy is focused on sending emails only to recipients who grant permission to receive your messages, then delivery challenges will be minimized. However, with mail servers and filters that can often be fickle in their processing, even solicited emails to willing subscribers can be turned away before reaching their intended inboxes. 

Here are some factors that prevent an email's delivery:

ISP-Blocked Emails

The most common form of ISP-blocked emails is for incoming mail. Many larger ISPs maintain internal blacklists of IP addresses that they’ve chosen to deny from communicating with their servers. This can be caused by customer complaints about too much traffic from particular sources and can lead to ISPs blocking IP ranges without any notification. In an event that is much less common, your ISP may even block your outgoing email traffic to certain ISPs.

Content Filters

There are a few different types of content filters that may result in emails not reaching their intended destinations. ISPs are often supported by anti-spam companies to help them to filter out distributed content that appears unsolicited based on either the content of the message or its source. Other times, ISPs will employ content filters that are created or adapted internally to scan for a variety of patterns they've associated with spam email, such as inserting spaces or punctuation within words that may normally trigger a red flag. There are also user content filters provided by almost every email client to help users filter certain words, phrases, or even domain addresses into their spam/junk mail folders.

Message Bounces

An email message may also be bounced back to the sender. A "soft bounce" is a temporary failure to send the email, meaning it wasn't delivered but can try to do so again in the future. This may be caused by the recipient's mailbox being full or their server being unresponsive. A "hard bounce" means that the email has been deemed permanently undeliverable. The cause of a hard message bounce could be that the recipient's email address is invalid or that a remote server is blocking your own server.


Before you can optimize delivery, you must understand the common causes of deliverability failure. We hope this post provided a good starting point.

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