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.
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 hard bounced email addresses from your ATS database. Herefish can also offer to indicate this hard bounce automatically in your ATS, should you prefer that as well.
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.
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 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.
Hard bounces = permanent
Soft bounces = temporary
In Herefish you can search against an email being labeled as a soft or hard bounce. Simply select the Bounce activity list logic and declare whether you'd want to search for ANY, HARD or SOFT bounces in the dropdown field.