Intro
Crono shows four email states: Valid, Accept-all, Unknown and Invalid.
This article explains the difference between an email that Bounced and an address marked Invalid, and why an address can turn red without anyone having emailed it.
1. “Bounced” Definition
Bounced means a send was attempted and the receiving server rejected it: the system receives a delivery failure message from the email provider, indicating that the email was returned (not delivered).
In this case, the email address is automatically marked as “Invalid” - red dot - to prevent future failed sends.
2. “Invalid” Definition
Invalid means Crono knows the address will not deliver, so it is not used for sending. A bounce always leads to Invalid, but a bounce is not the only route in - an address can be labeled as Invalid in several ways:
Caught by Bounce Shield: an address Crono found through enrichment is re-verified and comes back undeliverable, before any send is attempted. See section 3.
Triggered by a bounce event: if an email bounces, it is automatically marked Invalid.
Manually set by a user: a user can manually mark an address as Invalid.
Verified during manual input or CSV import: when manually added or imported, an address may be checked and flagged as Invalid.
3. Bounce Shield
Bounce Shield is Crono's check on email addresses it found through enrichment. Shortly after an address is found, Crono re-verifies it in the background with an external validation service. Addresses that come back undeliverable are marked bounced; the rest of the batch is confirmed as valid.
When that happens, every active contact in the workspace holding that address flips to email Invalid - the red dot - and the address is not used for sending.
This is why an address can turn red shortly after it was found, without anyone having emailed it. A red dot is not proof that a send bounced.
Bounce Shield runs on its own. It costs no credits, requires no action from you, and there is no setting, button or progress indicator for it - the only thing you see is the address changing state.
4. Possible Causes for Multiple Invalid Addresses
If many addresses are marked Invalid during a campaign or sequence, potential causes include:
Actually invalid email addresses that naturally bounced.
Bounce Shield flagged them after enrichment, before any send was attempted.
Provider-specific issues (e.g., Outlook) incorrectly returning messages as spam.
5. Recommended Actions
Review bounce logs to determine if errors are temporary or permanent.
Report unusual patterns (e.g., valid domains like Outlook returning messages).
Avoid re-sending to addresses already marked as Invalid.