Undeliverable emails; Error is [0x80004005-0x00000b22-0x00000501].

Hi Somerset Lesbian Network Admin

Based on your description, I understand that you were able to send the first two batches successfully (50 recipients each via BCC), but the remaining emails were returned as undeliverable with error [0x80004005-0x00000b22-0x00000501]. This helps narrow down the issue.

To better understand what is happening, could you please help confirm a few details:

• Are the recipients external email addresses (outside your organization)?
• Could you please share a full non-delivery report (NDR), including the detailed error and diagnostic information (similar to the screenshots below), via private message to avoid exposing any personal or organizational information?

This information will help identify whether the messages are being blocked by the sending service or by the recipient servers.

From similar cases, this type of issue often occurs when emails are sent to a large number of recipients within a short period (for example, using BCC for newsletters). In these situations, the activity may be identified as bulk or spam-like, which can lead to delivery failures after the initial messages are sent successfully.

Additionally, Exchange Online is not designed for bulk email distribution. Sending a high volume of similar messages may trigger service protection mechanisms, resulting in throttling or rejection of subsequent emails.

Please share the requested details when convenient, and I’ll be glad to assist you further.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

The undeliverable messages are consistent with Outlook.com or the recipient servers treating the bulk BCC sends as potential spam or otherwise refusing delivery.

Use these steps to improve delivery and work around the issue:

1. Check the NDR (bounce) details
   Open one of the “Undeliverable” or “delivery failed” messages and scroll down to the section like “Diagnostic information for administrators”. This section usually states the reason (for example, blocked, listed, message expired, or connection issues).
   If it indicates spam/blacklist or blocking on the recipient side, continue with the next steps.
2. Reduce recipients per message
   Even though 50 BCC recipients worked previously, bulk mail patterns can trigger filters. Reduce the number of recipients per message (for example, 20–25 per email) and resend.
3. Verify all recipient addresses
   A single bad or outdated address can sometimes contribute to delivery problems in bulk sends.
   • Confirm that each address is valid and still in use.
   • Remove any addresses that repeatedly bounce.
4. Check for blacklist or “blocked/listed in” errors
   If the NDR text includes wording like “blocked” or “listed in” with references to sites such as “spamcop,” “dynablock,” “blackhole,” or “spamhaus,” the recipient’s server is intentionally blocking the messages as suspected spam.
   In that case, contact the support team for the affected recipient’s email provider and request that the sending account or domain be unblocked.
5. Resend from Outlook.com web if using the old app
   If the messages are being composed in the old Outlook app and failing, try sending the same newsletter from Outlook.com directly. This can help rule out client-side issues.
6. If the error is “message expired,” “can’t connect,” or “connection timed out”
   When the NDR indicates:
   • “Can’t connect to remote server/message expired” or
   • “Connection timed out” or
   • “Out of memory” or “Resources temporarily unavailable”
     these are usually temporary issues on the receiving side. Wait a short time and resend. If the NDR says the recipient’s provider will keep trying to deliver, no action is needed.
7. **If the NDR is a ****@outlook.com / postmaster@microsoft.com message
   • Scroll to the diagnostic section to see the specific reason (for example, invalid address, attachment too large, or spam-related block).
   • If the problem is an invalid or mistyped address, correct it and resend.
   • If the problem is attachment size, reduce or remove attachments and resend.

If bulk sending continues to be flagged as spam despite valid addresses and smaller batches, consider using a dedicated mailing or newsletter service that is designed for large distribution lists and has better deliverability controls.

References:

• Sent email in Outlook.com comes back "delivery failed"
• Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com
• Outlook replies or forwards cause non-delivery error “Your message did not reach some or all of the intended recipients”