What is a catch-all domain?
Normally, when a sending server asks a receiving server about an address that does not exist, the receiving server answers with an error such as 550 user unknown. That error is what lets email verification spot invalid addresses without sending a message.
A catch-all (also called accept-all) server skips that check. It accepts mail for anything@company.com and sorts it out later, or delivers everything to one shared inbox. Companies set this up so they never miss mail sent to a misspelled name, and some security gateways do it to hide which mailboxes exist.
For anyone verifying a list, the result is the same: the server's "OK" says nothing about whether jane.doe@company.com is a real person.
Why catch-all addresses are a deliverability risk
- Hidden bounces. Some catch-all servers accept the message during delivery and send a bounce afterwards, which still counts against you.
- Silent black holes. Others accept and quietly discard mail to unknown names, so your sends look delivered but reach nobody, dragging down engagement.
- Spam traps. An accept-all domain can collect mail for addresses that were never handed out, which is how some traps are built.
- False confidence. A verifier that reports these as Deliverable makes a list look cleaner than it is.
How InboxSure detects catch-all servers
- Mailbox check. InboxSure connects to the domain's mail server and asks whether it will accept mail for your address, then disconnects without sending anything.
- Catch-all probe. On the same server it asks about a randomly generated address at the same domain, one that almost certainly does not exist.
- Verdict. If the server rejects the random address, it checks mailboxes properly and your address's result can be trusted. If it accepts the random address too, the domain is catch-all.
- Per-domain memory. The verdict is remembered for each domain, so a list with hundreds of contacts at one company does not hit its server again and again.
Large free providers such as Gmail, Outlook.com and Yahoo reject unknown addresses, so they are not probed and never reported as catch-all.
How results are reported
| Server behaviour | InboxSure result |
|---|---|
| Accepts your address, rejects the random one | Deliverable |
| Accepts both addresses | Risky, flagged catch-all |
| Rejects your address as unknown | Undeliverable |
| No clear answer, for example a timeout | Unknown |
When you download results you can include or exclude each status, so catch-all contacts can go into their own segment.
What to do with catch-all contacts
- Send to Deliverable addresses first. Build reputation with the contacts you know are real.
- Mail catch-all contacts in smaller batches. Watch the bounce rate of each batch and stop if it climbs.
- Prefer name-based addresses.
first.last@company.comon a catch-all domain is far more likely to be real than a random string. - Remove anything that bounces immediately after each send.
- Use engagement. After a few sends, drop catch-all contacts that never open or reply.
Want the full background? Read What Is a Catch-All Email Address?, or check a domain's mail servers with the free MX lookup.