Soft Bounce vs Hard Bounce

At Mercury Flight, there are two types of bounces that you can receive – a soft bounce/block or a hard bounce.

A soft bounce means that the email address was valid and the email message reached the recipient’s mail server. However, it bounced back because:

The mailbox was full (the user is over their quota).
The server was down.
The message was too large for the recipient’s inbox.

At Mercury Flight, we continue to attempt to send these messages for up to 72 hours until the message is delivered. If a message is continuously deferred for 72 hours, we convert these addresses to a block/deferral list. (A deferral list a not a suppression list.)

A hard bounce occurs when the message has been permanently rejected either because:

The email address is invalid.
The email addresses doesn’t exist.

At Mercury Flight, we add these hard bounced addresses to a suppression list. What this means is that even if you send a message through us for that user, we will not even try to deliver to that address, because we know it’s no longer good. Continuing to try to send to a known bad address will harm your reputation with the receiver, so we prevent that.

Bottom Line: Think of soft bounces as blocks that are a short term issue–you don’t need to permanently take these addresses off of your list. However, hard bounces are either invalid or non-existent addresses that should be removed immediately.

Reducing Your Bounces

The best way to reduce the number of bounces is by following some key email deliverability best practices. This includes the following:

Maintain good list hygiene: Purge your list regularly of invalid emails and non-responders. High bounce rates can affect your sender reputation so keeping your lists clean will go a long way to achieving higher email delivery rates.
Monitor your email delivery: Track your email delivery rates by paying close attention to your bounce rates as well as your response rates. With regular monitoring, you can catch potential failures before they happen or before they do too much damage.