When an email is not delivered to the intended recipient(s), it’s most commonly known as a ‘bounce’. When an email bounces, the receiving email server sends a notification, known as Non-Delivery Report (NDR), back to the sender to inform them that the email has not been delivered. The notification will include important information to help you identify the reason for the email delivery failure such as time and date the message bounced; the mail server that bounced it and the RFC code* with the reason for the bounce. A bounce usually falls into two categories: hard and soft bounces.
Hard Bounces
A hard bounce is an email which is considered to be ‘permanently unavailable’. They are often caused by sending emails to unknown or non-existent domains which more often than not results from questionable data sources or spam and it can be a cause for an email address to be blacklisted. Most commonly hard bounces happen when:
- You’re sending to inactive or non-existent email addresses;
- You’re sending to non-existent domain names; or
- There’s a blocked delivery by the recipient email server.
Soft Bounces
A soft bounce is an email which is considered to be ‘temporarily unavailable’. They often occur when the server rejects the email due to temporary conditions. Usually, there are three to five attempts made sending emails considered as soft bounces before they turn into hard bounces. The most common reasons soft bounces occur are:
- Recipient’s mailbox is full
- Temporary domain failure; or
- An account being disabled or inactive.
Nonetheless, soft bounces can also be the result of block bounces where the email server is rejecting the email due to filtering issues. Such as:
- Previous complaints from a user;
- Sender’s domain or IP address are found on a blacklist;
- Content spam score is too high;
- Problems with the authentication of the sender and/or sending source; or
- Blocked URL.
Reducing Bounces
The best way to reduce the number of bounces is by following some key email deliverability practices such as:
- Maintain clean email list: check your lists regularly in order to ensure there are no invalid emails or non-responders;
- Use double opt-in: by sending a confirmation email to users subscribing to your email list(s) you ensure that said user’s email addresses are not only valid but they, in fact, want to receive your emails;
- Monitor email delivery: track your email delivery rates by paying attention to not only the bounce rates but the response rates as well. When you regularly monitor the deliverability, you can notice potential failures even before they happen or before they do too much damage.
Fix: Double check the recipient’s email address to ensure it’s correct and try resending the message. It might’ve been a one-time issue. If the error persists, contact the recipient to check if their email address is correct. In case the issue still continues, contact the mail service provider for help.
Bounces, especially hard ones, are damaging the sender reputation. If you have a high bounce rate, it means you probably have a lot of non-existent email addresses in your database. By sending to non-existent email addresses you can hit a spamtrap. In order to avoid harming your sender reputation, we recommend reducing your bounces.