What defines “to keep”?

A "to keep" address is one you can confidently send to without hesitation. It's the green category in your validation results: no risk signals, no bounce history, no engagement concerns. These are the addresses that make your list worth having.

The criteria that define a keep-worthy address:

• Valid technical status: The address passes validation. The domain resolves, it has MX records, and the server confirms the mailbox exists (not just catch-all). No syntax errors, no domain red flags.
• Positive or neutral engagement history: The subscriber has opened or clicked at least once within a reasonable window (90-180 days is typical). Or they're new enough that you haven't had a chance to establish a pattern yet. Either way, there's no disengagement signal actively working against you.
• Clean acquisition source: You know how this address got on your list and it came in through a legitimate process. Double opt-in is the clearest signal. Single opt-in from a reputable form is acceptable. Imported from a purchased list is not, regardless of how the address validates.
• No complaint or suppression history: The address hasn't bounced, complained, or unsubscribed previously. A clean record.

Worth noting: an address that validates as "valid" doesn't automatically qualify as "to keep." A technically valid address from a purchased list is still a list problem because of the source. Validation status and keep-worthiness aren't the same thing. The source and engagement history matter alongside the technical checks.

The inverse is also true: an address that validated as catch-all but has opened every email you've sent in the past six months belongs in the keep column. Good behavior overrides uncertain validation status when the evidence is strong.

Keep-tier addresses are your core audience. Protecting them means maintaining hygiene on the rest of the list so they don't get buried in the noise of bad addresses hurting your deliverability.