Bounces - Online Help | Zoho Campaigns

Bounces

In short: a bounce is an email the receiving server wouldn’t accept. Soft bounces are temporary and retried; hard bounces are permanent and should be removed and never re-mailed. Bounces are one of the clearest signals of list quality, so keep your total bounce rate under about 2% (hard bounces under 0.5%) by verifying and cleaning your list before you send.
Bounces occur when sent emails can’t be delivered. Typically the receiving server returns an error message explaining why. To understand where bounces fit, it helps to walk through what happens when you send an email.

Bounces – An infographic.

How are bounces generated?

Once your email is ready and you click Send, the system looks up the recipient’s domain and mail server and makes contact. The receiving server then decides whether to accept the message. If it accepts and delivers, that’s a success. If the server finds the address invalid or non-existent, it returns a permanent failure — a hard bounce. If the address exists but the server can’t accept the message right now (for example, the mailbox is full), it returns a temporary failure — a soft bounce.

The two behave very differently, and they call for opposite responses, so it’s worth treating them separately.

Soft bounce

A soft bounce is a temporary delivery failure — the receiving server replies with a 4xx code. Common causes include a full mailbox, a server that’s down or slow, a message that’s too large, greylisting, or temporary reputation throttling. Because the problem may clear, the message isn’t abandoned: Zoho Campaigns retries delivery several more times, and only if it keeps failing is the address reclassified as a hard bounce. Persistent soft bounces to the same address are a sign it’s effectively dead and should be suppressed.

Hard bounce

A hard bounce is a permanent delivery failure — the receiving server replies with a 5xx code. The address is added to your Bounced list automatically and should never be emailed again. Frequent hard bounces drag down the reputation of your sending domain and IP fast.
Common hard bounces include:
    • The recipient email address doesn’t exist.
    • The domain name doesn’t exist.
    • A typo makes the address invalid (for example, gnail.com instead of gmail.com).
    • The recipient’s server has blocked delivery.
    • The address is inactive.
    • The email account has been closed.
Hard bounces are serious. A rising hard-bounce rate badly hurts your reputation with ESPs and ISPs and can lead to blocklisting, so hard-bounced addresses must be suppressed immediately — don’t retry them. In short: hard bounces are permanent deliverability failures.

Watch your bounce rate

Bounce rate is calculated as bounced emails ÷ emails sent × 100, and it’s a direct read on list quality. Commonly cited healthy thresholds are a total bounce rate under 2% and a hard-bounce rate under 0.5%; above roughly 5%, many ESPs will flag or suspend an account. This matters more than it used to: since the 2024 Google and Yahoo bulk sender requirements, mailbox providers scrutinize sending quality closely, and a high bounce rate is one of the fastest ways to erode inbox placement.

Where to find bounced contacts

You can find hard-bounced contacts under the Bounced Contacts view in the Audience section:
  1. Navigate to Audience -> Contacts.
  2. Click the More Actions (3-dotted) button in the top bar.
  3. Select the Bounced view to show the hard-bounced contacts.
You can also follow these steps to find the bounce reason:
  1. Go to Campaigns -> Emails.
  2. Open the relevant email campaign by clicking it.
  3. In the Overview tab that appears, click the percentage under Soft Bounces or Hard Bounces. You will see the list of contacts that suffered from a soft bounce or hard bounce (depending on what you clicked).
  4. You will see the Bounce Reason column which will show why the contact got bounced. Click the reason to open a slide-in window that shows additional details for the bounce. 
This shows you the bounce reason in Zoho Campaigns.

How to avoid bounces

Almost every bounce traces back to list quality, so the fixes are mostly about who you add and how you keep the list clean.
  • Verify addresses before you send. Pre-send email verification is the single most effective way to keep bounces low, since data decays steadily over time.
  • Don’t use purchased mailing lists.
  • Use double opt-in to keep invalid addresses out of your list in the first place.
  • Clean your list regularly with an email-validation service, and suppress addresses that have hard-bounced or persistently soft-bounced.
  • Avoid blocklisted URLs in your content; keep subjects and copy honest (modern filters weigh reputation and engagement far more than individual "spam words").
  • Keep abuse complaints down — Google and Yahoo want your complaint rate below 0.1% — and avoid role/group addresses, which bounce and complain more often.
  • Maintain a steady sending volume and a healthy frequency; warm up new domains and IPs.
  • Authenticate your sender domain with SPF, DKIM, and DMARC.
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