Hard bounces vs soft bounces
Mail servers reply to every delivery attempt with a three-digit code defined in the SMTP standard, RFC 5321. Codes starting with 5 are permanent failures: the server is saying don't try again. Codes starting with 4 are temporary failures, and the sending server will usually retry for a while before giving up.
- Hard bounce (5xx). The mailbox doesn't exist, the domain has no mail server, or the receiver rejected you outright. Never send to that address again.
- Soft bounce (4xx). The mailbox is full, the server is busy or rate-limiting you, or the message is too large. It may deliver on a retry.
- Blocks. Some rejections are about you, not the address: a server refusing mail because of your domain's reputation or missing authentication. These look like bounces but point to a sender reputation problem.
Reading an email bounce back message
The bounce back (also called a non-delivery report) is the automatic email you receive explaining the failure. It usually includes the SMTP code and an enhanced status code from RFC 3463, like 550 5.1.1, where 5.1.1 means the destination mailbox address is invalid. The text after the code varies by provider but often says "user unknown" or "address not found".
If the code starts with 4 and the message says the mailbox is full or try again later, wait. If it starts with 5 and mentions the user or address, remove it. If it mentions policy, spam or authentication, check your domain setup with the SPF, DKIM and DMARC checker before sending more.
Why bounces matter for outreach
Mailbox providers don't publish a bounce-rate cutoff, but they do notice a sender who keeps hitting addresses that don't exist. That's the pattern of someone guessing or buying lists, and it drags down the reputation of the domain sending it. For cold outreach, where every recipient is new, a few careless sends can push a young mailbox toward the spam folder.
The catch-all problem makes it harder: some domains accept every address at first and bounce later, so a verifier can't confirm the mailbox exists.
How to keep bounces low
- Verify every address before the first send. Use the address published on the site when you can; the email permutator and a verifier help when you only have a name. Finding an editor's email covers the order to try.
- Suppress any address that hard-bounces, permanently, across every campaign.
- Pause sending when bounces rise suddenly, and find out why before you resume.
- Check the domain has a working mail server with the MX lookup before writing to it.
How BacklinkGenie handles bounces
Every contact is checked before it's used, and addresses that fail are never emailed. A hard-bounced address goes on a suppression list so it isn't contacted again, and if a mailbox's bounces climb, sending from it pauses automatically until the cause is found. All of this runs on a separate outreach domain, so your company email is never exposed.
Example
Guessing chris@blog.example without checking it, and getting a "user unknown" reply, is a hard bounce.
Putting it into practice
Where to use it: Email permutator and Email deliverability for link outreach. For the longer version, read how to find an editor's real email address.