Every bounce starts with a reply code. Your sending server connects to the recipient's mail server and names each recipient. When the server refuses, it answers with a three-digit code and a short line of text, and your email platform turns that answer into the bounce you see. Codes starting with 5 are permanent, which makes them a hard bounce, and codes starting with 4 are a soft bounce.
The address doesn't exist. This is the classic hard bounce, a 550 5.1.1 user unknown: the person left the company, someone mistyped the address, or it was never real. The domain can be dead too, with no DNS record or no mail server behind it. Hunter found 2.3% of verified addresses turned invalid within eight weeks (Hunter, 4 November 2025), so this cause grows on any list you leave alone.
The mailbox can't take mail right now. A 552 mailbox full means the owner has run out of space, and a disabled account also returns a 5xx code. A full mailbox can recover once its owner clears space, so re-check those rows after 30 days rather than dropping them for good.
Your mail was blocked. A 550 5.7.1 or a 554 transaction failed usually points at you, not the address: a blocklist listing, failed SPF or DKIM, or a weak sender reputation. You fix these on your own domain and IP, because the address itself is fine.
The server said try later. Greylisting, rate limits and throttling all return 4xx codes, and Microsoft rate-limits aggressively (see the Microsoft throttling fix). Your platform retries for a while before it gives up. When the same address soft-bounces send after send, treat it as the soft bounce loop it is.
You can stop the address-side bounces before you hit send. A verification check asks the server about each mailbox at the RCPT TO step and closes the connection before any message goes out. ZapBounce marks each row valid, invalid, catch_all or unknown, charges $25 for 10,000 checks, and never bills unknowns. If your bounce rate is already climbing, start with why a bounce rate runs high.
Related questions
Why do emails bounce back when the address is correct?
A correctly spelled address still bounces when the mailbox is full, the server throttles you, or your domain is on a blocklist. The code then starts with 4, or reads 550 5.7.1 or 554, and the fix sits on your sending side. Check your authentication and reputation before you resend.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is permanent: a 5xx code saying the address or domain won't take mail. A soft bounce is temporary: a 4xx code for a busy server, greylisting or throttling. Remove hard bounces after one failure, and watch soft bounces that repeat on later sends.
Do bounced emails hurt my sender reputation?
Yes. Email platforms track how often you mail addresses that don't exist. Amazon SES reviews an account at a 5% bounce rate and may pause sending at 10%, per its enforcement FAQ read 24 September 2026.
How do I stop emails from bouncing before I send?
Verify the list first to remove addresses that don't exist, since an SMTP check finds them without sending a message. ZapBounce costs $5 for 1,000 checks and never bills unknown results. Blocks aimed at your sender need authentication and reputation fixes instead.