In general, a Bounce means an email can't be delivered to an inbox. "Hard" and "soft" designate the two groupings of failures; one is pretty permanent, the other might not be.

Hard Bounce

A hard bounce indicates a permanent reason an email cannot be delivered. There are lots of reasons that an email could be a hard bounce, but the core of it is that it's a permanent failure. You should remove all of these addresses from your list, Herefish will do that automatically and also indicate the bounce in your ATS. 

Some common reasons for a hard bounce:

  • Recipient email address does not exist.
  • Domain name does not exist.
  • Recipient email server has completely blocked delivery.

Soft Bounce 

A soft bounce is an email that couldn't be delivered because of temporary reasons.  Often email providers will continue to try to deliver the email a few times. Herefish will automatically remove treat an email that gets 3 soft bounces, as a hard bounce. 

Some common reasons for a soft bounce:

  • Mailbox is full (over quota).
  • Recipient email server is down or offline.
  • Email message is too large.

Ideally your bounce rate will be under 2.5%. If it starts to go higher than that, you'll likely start to have issues with delivery. 

In short:
Hard bounces = permanent
Soft bounces = temporary

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