When an email you send bounces, Zoho Campaigns categorises it as either a hard bounce or a soft bounce, based on the error code your recipient's mail server returns.
A hard bounce is a permanent delivery failure, typically carrying a 5.x.x error code. Recipients are added to your Bounced list immediately, and Zoho Campaigns won't retry sending to them. Common causes include the email address not existing, the domain not existing, the address being invalid or malformed, the recipient's server permanently blocking your domain, or the address having been deleted or deactivated.
A soft bounce is a temporary delivery failure, typically carrying a 4.x.x error code. It doesn't mean the address is invalid, so Zoho Campaigns retries delivery automatically. Common causes include a full mailbox, a temporarily unavailable mail server, oversized content, or content flagged as spam.
If a campaign's bounce rate exceeds 5% while sending, Zoho Campaigns automatically pauses it and notifies you, resuming after 24 hours. If the bounce rate still exceeds 5% after resuming, the campaign is permanently cancelled. Keeping your list clean is directly tied to whether your campaigns can send at all, not just a reputation concern.