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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →SMTP 250 does not always mean an email reached the recipient’s inbox. A 250 reply after RCPT TO accepts a recipient path; a 250 after the end of DATA means the receiving server accepted the message for delivery or relay. A later delivery attempt can still fail and generate a bounce. The title’s 50 sent and 12 bounced are not enough to identify the cause.
What an SMTP 250 actually confirms
SMTP is a command-and-reply exchange. During a typical transaction, the sending system identifies the sender with MAIL, specifies recipients with RCPT, and sends the message content with DATA. A server can reply 250 at more than one stage, and each reply means something different.
| Where the 250 appears | What it confirms | What it does not confirm |
|---|---|---|
After MAIL |
The server accepted the sender identification for the transaction. | That a recipient was accepted or the message was delivered. |
After RCPT TO |
The server accepted that recipient path for the transaction. | That the message content was accepted or reached an inbox. |
After the end of DATA |
The server positively completed the message transaction and accepted responsibility for delivery or relay. | That downstream delivery succeeded, that the message reached the inbox, or that anyone read it. |
RFC 5321 makes the distinction explicit: “When the receiver-SMTP accepts a piece of mail (by sending a ‘250 OK’ message in response to DATA), it is accepting responsibility for delivering or relaying the message.” That statement refers specifically to the positive completion reply after DATA, not every 250 in a transaction. See RFC 5321, especially sections 3.3, 4.2.5, 6.1, and 4.5.4.
How a bounce can follow a successful handoff
A server that accepts a message after DATA may need to pass it to another system before it reaches the destination. If a later delivery attempt encounters a temporary problem, the receiving system can retry. If delivery ultimately fails permanently—or temporary failures continue until retry attempts are exhausted—the system should notify the original sender using the address given in the SMTP MAIL command. The earlier 250 and the later bounce can therefore both be accurate: one records acceptance by a server; the other reports a subsequent failure to deliver.
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“Accepted by the next SMTP server” is the right description only when the sending transcript shows a positive completion reply after DATA. That is a handoff, not proof of inbox placement.
How to investigate the 12 bounces
- Open each non-delivery notice. Gmail users can look for a message from Mail Delivery Subsystem, often titled “Delivery Status Notification (Failure).” Keep the complete diagnostic details rather than relying on the subject line. Google’s guide, Fix bounced or rejected emails, describes common causes and where to find the error.
- Group notices by recipient and error. For each message, record the recipient address and domain, time, full enhanced status code, and diagnostic text. A total of 12 does not establish that all failures had the same cause.
- Find the exact 250 in the sending log. Check which command preceded it:
MAIL,RCPT TO, or the end ofDATA. Preserve any queue or message ID and use it to connect the accepted transaction to the later bounce. - Classify the full enhanced status code. Enhanced codes use the form
class.subject.detail. Microsoft explains that class 4 indicates a temporary error and class 5 a permanent one; the subject identifies a broader area such as addressing, mailbox, mail system, network or routing, protocol, or security and policy. The class alone is not a full diagnosis. See Microsoft’s documentation on DSNs and NDRs and the IANA Enhanced Status Codes Registry. - Follow the specific diagnostic evidence. Depending on what the notice says, check whether the recipient address exists, whether the message was rejected as spam or temporarily, whether a sending limit applies, and whether the recipient mailbox is temporarily unavailable or full. The registry also contains authentication-related entries, including SPF validation and reverse-DNS validation failures. Investigate those only if the returned code or text points to them.
Compare failures before choosing a fix
When several bounces are involved, compare them by SMTP stage and reply, temporary or permanent class, enhanced-code subject, recipient domain or system, and the diagnostic text. A temporary delivery problem calls for a different response from a permanent address error or a security-policy rejection. Grouping the notices this way helps reveal whether failures share a cause without inferring one from the count.
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- Different recipient domains, different text: treat the notices as separate cases until the evidence shows a shared issue.
- Same domain and similar temporary errors: the recipient system may be reporting a temporary delivery condition; use the full notice and subsequent delivery status to understand whether it clears.
- Policy or authentication wording: verify the specific sender or message condition indicated by the response before changing configuration.
What the title does—and does not—tell you
“I sent 50 emails and got 12 bounces” describes a count, not a diagnosis or a general bounce rate. Without the SMTP command/reply sequence, recipient domains, complete bounce notices and enhanced codes, and relevant sender or relay configuration, the cause could be an invalid recipient, recipient-side limits or storage, authentication, policy, or another downstream condition. The available details do not establish which explanation applies.
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