An Email Bounce, or Non Delivery Notification (NDN) is electronic mail that is returned to the sender because it wasn't able to be delivered for any of a number of reasons.
Depending on
how you have your email servers configured, bounces sometimes get returned to the sender, the person who sent the email. Sometimes they get set to the postmaster or admin account and a
typical email sender will never see the bounce. Sometimes the intended receiptant email servers, doesn't even send a bounce reply to non-deliverable addresses.
There are two kinds of e-mail bounces, hard bounces and soft bounces.
Hard Bounces
Email that are permanently bounced back to the sender because the address is invalid or the mailbox doesn't exist.
Below are a few examples of hard bounces:
| Code | Definition |
| 5.1.1 | Invalid Mailbox |
| 5.2.0 | Message Identified as Spam |
| 550 | Mailbox Not Found |
| 554 | No Valid Recipients |
| 551 | Not Our Customer |
Soft Bounces
Soft bounce email is recognized by the recipient's mail server but is returned to the sender because the recipient's mailbox is full or the mail server is temporarily unavailable.
Below are a few examples of soft bounces:
| Code | Definition |
| 5.2.2 | Mailbox is full |
| 452 | Mailbox Size Limit Exceeded |
| 552 | User Exceeds Storage Quota |
| 554 | Relay Access Denied |
|
Processing Bounces
Bounces are typically handled by a program when sending email to a distribution list and most email distribution list vendors include this capability.
Some bounce handlers can retry sending a soft bounce at a later date, can unsubscribe the addressee from your distribution list or take other actions.
You can also process your bounces manually which is a very tedious exercise with a large distribution list. Bounces contain instructions, or links,
that you have to click in order for the email to be delivered and your company to be added to the addressee's whitelist.
It's important to remove all
hard bounces from your email list. If you continually have too many bounces to certain ISPs it can get you added to their blacklist.
You should also remove
soft bounces that occur repeatedly.
Below Are Some Industry Average Email Bounce Rates
Source: Unlock The Inbox