The four reasons verified addresses still bounce

1. 01

   1. Time decay: the address was verified too long ago.

   Mailbox turnover is real. B2B mailboxes get deactivated when the employee leaves. Consumer mailboxes get abandoned. A verdict from six months ago is not the same as a verdict from today. Reverify lists that are older than 60 to 90 days before major sends.

2. 02

   2. Recipient server policy changed between verify and send.

   Corporate mail servers roll out spam-filter updates weekly. An address that accepted mail at verification time may be rejected today because your sending domain matched a new filter rule. This is invisible to any verifier.

3. 03

   3. Your sending IP has poor reputation.

   Some recipient servers do not even open the envelope on senders with a low IP reputation score. The bounce reads as "user unknown" or "550 mailbox unavailable" even though the mailbox exists. Google Postmaster and Microsoft SNDS will tell you if this is happening.

4. 04

   4. The address sits on a catch-all domain.

   Catch-all domains accept every address at the SMTP layer, then decide later whether to route or reject. A verified catch-all address might be delivered to a spam folder, silently discarded, or bounced asynchronously days later. Every verifier flags catch-all separately for this reason.

How to tell which one is causing your bounces

What to do about it

• Reverify lists older than 60 to 90 days before major campaigns
• Segment catch-all addresses separately and monitor their engagement (opens, clicks) as a proxy for actual delivery
• Monitor Google Postmaster (postmaster.google.com) for IP and domain reputation
• For B2B lists, resample known-departed employees quarterly. LinkedIn scrape is cheap signal.
• If bounce spikes on a single domain, contact that domain owner and ask about recent policy changes

Where credit-back applies

KillBounce credit-back covers hard bounces on addresses returned as deliverable within a 30-day window from verification. Bounces from time decay past 30 days, catch-all asynchronous rejects, and reputation-driven refusals are outside the window because the verification verdict was correct at check time.