Three questions bring people here: what catch-all means, how a hard bounce differs from a soft one, and how many bounces are too many. The answers are in the mail standards, not in vendor articles, and they separate two problems most pages mix together: bounces, which mean the address failed, and spam complaints, which mean a person objected.
What does email verification check?
A verifier runs the first half of a real delivery and stops before sending anything. It checks the address syntax, looks up the domain's mail servers in DNS, connects to one, and names the recipient. The server's reply to that recipient is the result. RFC 5321, the SMTP standard, defines what those replies mean: a code starting with 2 is acceptance, 4 is a temporary failure, and 5 is a permanent one.
A 250 reply means the server accepts the mailbox; a 550 with the enhanced code 5.1.1 means it does not exist. Some servers give no useful answer at all, and that is where verification stops being able to help.
What is a catch-all domain, and why is it a problem?
A catch-all domain is configured to accept mail for any address at that domain, real or not. The server replies 250 to anna@ and to xyz123@ alike. Some then deliver to a shared inbox, and some bounce the message later, after accepting it. A verifier asking the server gets the same yes for every address, so it cannot tell a real mailbox from a guess.
Catch-all is therefore a third result, not a pass. Treat it as unknown. Send to catch-all addresses only when the address came from a source you trust, such as a pattern confirmed elsewhere in the same company, keep them a small share of any send, and watch bounces from that domain closely.
The reply codes are defined by the standards, not by verification vendors. RFC 5321 sets the meaning of 2yz, 4yz and 5yz replies, and RFC 3463 defines the enhanced status codes such as X.1.1 (bad destination mailbox) and X.2.2 (mailbox full). Google lists the exact replies Gmail returns, including 550 5.1.1 and 452 4.2.2, in its Gmail SMTP errors and codes reference. For complaint thresholds see Google's email sender guidelines.
Hard bounce vs soft bounce: what is the difference?
A hard bounce is a permanent failure: the receiving server answered with a 5xx code, such as 550 5.1.1 for a mailbox that does not exist. A soft bounce is temporary: a 4xx code, such as 452 4.2.2 when the mailbox is full. The enhanced codes after the main code come from RFC 3463, where X.1.1 means a bad destination mailbox address.
| Code | Meaning | What to do |
|---|---|---|
| 250 | Accepted: the server takes the recipient | Send |
| 550 5.1.1 | Mailbox does not exist (hard bounce) | Remove the address; never retry |
| 5xx, other | Permanent rejection, such as policy or block | Stop; investigate before sending more to that domain |
| 452 4.2.2 | Mailbox full (soft bounce) | Retry later; remove after repeated failures |
| 4xx, other | Temporary failure, such as rate limiting or server busy | Retry later at lower volume |
| 250 for every address | Catch-all: server accepts anything | Treat as unknown; send cautiously or verify another way |
Codes from RFC 5321, RFC 3463 and Google's published Gmail SMTP error list.
What do mailbox providers do when bounces and complaints rise?
They throttle, then filter, then reject. A sender that keeps hitting mailboxes that do not exist looks like a sender working from an old or bought list, and reputation drops for the domain and the sending server. Bounces and complaints are separate signals, and the published thresholds apply to different things.
- Spam complaint rate: Google's sender guidelines ask senders to keep the rate reported in Postmaster Tools below 0.3 %, and recommend below 0.1 %. This counts people marking mail as spam, not bounces.
- Hard bounce rate: there is no general standard. The 8 % limit often quoted comes from Microsoft's email policies for one product, Dynamics 365 Customer Insights, and governs sending from that product only.
- Practical rule: a cold sender should treat any rising bounce rate as a list problem to fix before the next send, not as a number to stay under.
What happens after a bounce in a sequence?
A hard bounce should end the sequence for that contact at once. No follow-up step will reach a mailbox that does not exist, and each retry adds another failure to your reputation. The address should be marked invalid wherever it lives, including the CRM, so the next campaign does not pick it up again.
A soft bounce usually deserves a later retry, since a full mailbox or a busy server can recover. Repeated soft bounces from the same address over several days behave like a hard bounce and should be treated as one. Bounces also feed the rest of your data: a hard bounce at a contact who used to work there is often the first sign of a job change.
In Leadspicker an email verification costs 2 credits, which is €0.02. The in-app price list is the authoritative source and prices can change.
Frequently asked questions
What is a catch-all email address?
An address on a domain whose server accepts mail for any name, real or not. Verification cannot confirm it, because the server says yes to everything, so treat it as unknown rather than valid.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent failure with a 5xx code, such as 550 5.1.1 for a mailbox that does not exist; remove the address. A soft bounce is temporary, with a 4xx code such as 452 4.2.2 for a full mailbox; retry later.
What is a safe email bounce rate?
There is no general standard. The 8 % figure often quoted is a limit in one Microsoft product. Google's published threshold, 0.3 %, is for spam complaints, not bounces. For cold email, treat any rise in hard bounces as a list problem to fix before the next send.
- IETF, RFC 5321: Simple Mail Transfer Protocol
- IETF, RFC 3463: Enhanced Mail System Status Codes
- Google Workspace Admin Help, Gmail SMTP errors and codes
- Microsoft Learn, Email policies and suspension standards (Dynamics 365 Customer Insights)
- Google, Email sender guidelines
- GTM playbook for 2026 by Leadspicker, chapters: How credits work
