Emails being returned by postmaster

Question

Outgoing emails being returned from postmaster saying recipients email rejected it, been sending same emails monthly for years with no issues

Answer 1

Hi Joel,

Can you please help confirm if test email that will be sent to your own email is working fine? Also, is the issue with sending email to the recipient in different email client or on your webmail persist? If the answer to both is yes then the issue is not with your setup but with the recipient’s mail server. The “postmaster” return message means their system is rejecting your emails outright. Since your account can send and receive normally, the test proves your side is functioning. The consistent rejection across platforms shows the block is happening at the recipient’s end, likely due to new filtering rules, blacklisting, or stricter spam controls. In this case, the resolution must come from the recipient’s IT team or email provider, who will need to whitelist your address or domain.

Answer 2

The postmaster message indicates the recipient’s mail system rejected the message, even though similar emails worked in the past. The next steps depend on the specific error text inside that postmaster/non‑delivery report (NDR).

1. Open the postmaster email and scroll down to the detailed error section
   • Look for a section labeled similar to “Diagnostic information for administrators”.
   • This section contains the SMTP error code and reason (for example, address doesn’t exist, mailbox full, message blocked as spam, or attachment too large).
2. Check for common causes and fix them
   • Invalid or changed address: The recipient may have changed addresses or their mailbox was removed.
     • In a new message, delete the recipient from the To field.
     • Start typing the address, select it from Auto‑Complete, then press Delete to remove the cached entry.
     • Manually retype the full, correct email address and send again.
   • Attachment size: If the NDR mentions size limits and the message has attachments, the recipient’s server may now enforce stricter limits.
     • Reduce attachment size or upload the file to a cloud service (such as OneDrive) and send a sharing link instead.
   • Forwarding issues on recipient side: If the NDR mentions forwarding or another destination address, the recipient may have a broken forwarding rule.
     • Ask the recipient to check their mail rules and forwarding settings for any invalid destination addresses.
3. If the error is a relay or spam‑related rejection
   • If the NDR shows codes like 550 or 553 with wording such as relaying denied or not in allowed rcpthosts, the recipient’s server does not accept the message from the sending system.
   • If the NDR or the postmaster guidance points to spam or reputation issues, the recipient’s system may now treat these messages as spam even though they were accepted before. In that case, the recipient’s admin or mail provider must adjust their filters or allowlist the sender.
4. Resend after correction
   • After correcting the address, attachment, or any identified issue, resend the message from Outlook.com/Outlook on the web.
   • If the same NDR continues and the error clearly comes from the recipient’s system (not Outlook.com), the recipient or their email administrator must resolve it on their side.

If the postmaster message itself looks suspicious (odd sender address, unexpected links, or attachments), validate it as described in the Outlook phishing guidance before acting on it.

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References:

• Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com
• I receive a 550, 553, or relay-prohibited error when sending email messages
• Sender Support in Outlook.com