Bounces and delivery errors explained
What a bounce looks like in Private.Ki, how to read the SMTP code inside it, which errors the app shows before a message even leaves, and what to do about each.
Mail between Private.Ki users does not bounce: it is placed straight into the recipient's mailbox. Mail to an outside address travels over ordinary SMTP, and the receiving server can refuse it. When that happens the refusal comes back to you as a bounce — also called a non-delivery report or Mail Delivery Failure. This article explains how to recognise one, how to read it, and what the codes inside it mean.
Two kinds of failure
Refused before leaving. Private.Ki checks external messages against public blocklists before handing them to the mail relay. If a recipient domain, the subject or a link in an unencrypted body is listed, sending stops and the composer shows Message blocked: Your message contains content flagged as spam. Nothing was sent and nothing arrives at the recipient. For a scheduled message the same refusal shows as an error on the message in Sent. See How to avoid spam for what is checked.
Refused by the recipient's server. Everything else. The message left our server and was accepted by the relay; minutes, hours or occasionally days later the other side's answer arrives as a bounce.
How a bounce looks
A bounce is an ordinary email, so it arrives in your Inbox like any external message:
- The sender is the mail system rather than a person — typically Mail Delivery System or MAILER-DAEMON@….
- The subject is along the lines of Undelivered Mail Returned to Sender, Delivery Status Notification (Failure) or Mail delivery failed.
- It shows the open padlock, because it was not encrypted on the way in — that is normal.
- The body names the address that failed and quotes the reply from the remote server, including a three-digit code. Your original message is usually attached.
An out-of-office or auto-reply is not a bounce: your message was delivered, and a robot answered.
Reading the code
The important part is the three-digit code and, often, a dotted enhanced code such as 5.1.1. The first digit tells you whether trying again can help.
| Code | Meaning | What to do |
|---|---|---|
550 5.1.1 — User unknown, No such user |
The address does not exist. | Check the spelling; ask the person for their current address. |
552 or 5.2.2 — Mailbox full, over quota |
The recipient cannot receive anything right now. | Reach them another way; try again later. |
550 5.7.1 — policy, spam, rejected |
Their server classified the message as unwanted. | Remove links and attachments they might object to, or ask them to allow your address. If it persists, tell us — it may concern our server's reputation. |
554 — Service unavailable, blocked |
Their provider refuses mail from our server or domain. | Please report it to support with the bounce attached; the recipient may also be able to whitelist private.ki. |
552 — message size exceeds limit |
Too large for their server. | Send fewer or smaller attachments, or share the file another way. Private.Ki's own limit is 25 MB per file. |
4xx — 421, 450, 451, try again later, greylisted |
Temporary. DNS trouble, a busy server, or deliberate delay of first-time senders. | Nothing yet. The relay keeps trying; a delayed warning means it has not given up. A bounce arrives only if all retries fail. |
5.4.x — host not found, domain does not exist |
The part after @ is wrong. |
Check the domain spelling. |
Codes starting with 4 are soft: the message is still queued. Codes starting with 5 are hard: the attempt is over, and resending the same message will fail the same way until something changes.
Getting the exact text
Everything the remote server said is in the bounce. If you need to pass it to us or to the recipient's IT department, open the bounce and use ⋮ → Show original, then Download to save the whole thing as an .eml file. See View headers and the original message.
What our server sees
Cannot see
- The content of your original message if it was encrypted for the recipient
Can see
- That a message to an external address was handed to the relay, and to which domain
- The bounce itself, as it arrives from outside — like any external message it is then stored encrypted for you
Common questions
The app said "Sent" but the recipient has nothing and I have no bounce
Check the recipient's spam folder first; delivery to a mailbox is not the same as delivery to the inbox. Temporary failures can take a while before a bounce arrives. If nothing appears after a day, ask the recipient to confirm the address and try again.
I got a bounce for a message I never sent
Spammers forge sender addresses, and the bounce is sent to the forged address. Look at the quoted original: if it is not yours, delete the bounce. Your account has not been compromised by this alone.
Do messages to other Private.Ki users ever bounce?
No. If the address exists, the message is delivered immediately. If it does not, the composer shows Unable to send email because the following email addresses do not exist and disables Send until you fix the address.