Some Bulk emails coming back

Hi @Hans Schmidt,

I hope you’re doing well today.

I understand how inconvenient it can be when bulk emails that used to send without any issues suddenly begin bouncing or failing - especially when these messages are essential for keeping your recipients informed.

When bulk mail stops going through in Outlook on the web, it’s most commonly due to newly enforced sending limits, a temporary outbound block, or mail being flagged as high‑risk and diverted because it appears similar to bulk or spam traffic.

To move forward, here are the recommended steps your IT admin should review.

(If you’re not the IT admin, please share these with them):

1/ Check for tenant‑wide throttling or blocks

• Run a Message Trace (EAC > Mail flow > Message trace) to view delivery status such as delivered, deferred, or filtered as spam: Message trace in the new EAC in Exchange Online | Microsoft Learn.
• Review the Tenant External Recipient usage report to see if the organization hit outbound limits: Introducing Exchange Online Tenant Outbound Email Limits
• Check Restricted entities for any blocked users or connectors and only unblock once account security is verified: Remove blocked users from the Restricted entities page - Microsoft ...

2/ Review your sending limits and sending method

• Confirm per‑mailbox and per‑message limits (e.g., daily recipient caps or per‑message recipient limits): Exchange Online limits - Service Descriptions | Microsoft Learn
• Ensure no bulk mail is being sent from the .onmicrosoft.com domain, which has very low external recipient limits.
• Ensure no one is sending from .onmicrosoft.com for bulk mail; that domain is capped at ~100 external recipients/day tenant‑wide. Use the verified custom domain instead.

3/ Check outbound spam policy behavior

• Look for alerts such as “User restricted from sending email” or “Suspicious sending patterns”: Configure outbound spam policies - Microsoft Defender for Office 365
• If messages are routing through the High‑risk delivery pool, this will negatively affect deliverability. Adjust sending patterns and ensure proper authentication to prevent this: Outbound delivery pools - Microsoft Defender for Office 365.

4/ Verify domain authentication (SPF, DKIM, DMARC)

• Missing or weak authentication often results in bulk messages being throttled or flagged as suspicious.
• Ensure all three protocols are correctly configured for your sending domain.

Reference: Email authentication - Microsoft Defender for Office 365.

I hope this helps point you in the right direction. Please let me know how things go after these checks. I’m happy to assist further.

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Bulk or high-volume sending from Outlook on the web is limited by the service to protect against spam and abuse. When those limits are reached, some messages in a bulk send can start bouncing or be blocked, even if similar sends worked before.

Key points and checks:

1. Sending limits
   • Outlook.com and Microsoft 365 impose daily and per-minute sending limits, and also limit how many recipients can be in one message.
   • Newer or recently changed accounts can have lower temporary limits until they establish a good sending reputation. These limits increase automatically over time as credibility is established.
   • If limits are exceeded, some messages in a bulk send may fail or generate non-delivery reports (NDRs).
   • For consumer Outlook.com accounts, see the documented limits in Sending limits in Outlook.com.
2. Non-delivery reports (bounce messages)
   • When a bulk send partially fails, NDRs usually include a reason, such as:
     • Invalid email address
     • Recipient mailbox full
     • Server issues on the recipient side
     • Spam filters blocking the message
   • Review the NDR text for each failed recipient to see whether the issue is per-recipient (e.g., invalid address) or due to throttling/spam filtering.
3. Content and spam filtering
   • Bulk messages with similar content, links, or attachments can be flagged as spam by recipient servers.
   • If recipients report messages as junk, or if the content resembles typical spam, more of the bulk send may be blocked.
   • Ask affected recipients to add the sending address to their safe senders list where appropriate. For Outlook.com recipients, they can use Safe Senders in Outlook.com.
4. Attachments and size
   • Large attachments can cause individual messages to fail, especially in bulk sends.
   • Outlook.com cannot send messages with attachments larger than 25 MB. For Outlook desktop, attachments larger than about 20 MB can cause messages to get stuck in the Outbox and block subsequent sends.
   • If bulk emails include large files, reduce attachment size or send links instead.
5. Steps to improve bulk sending success
   • Split large recipient lists into smaller batches and send over time instead of all at once.
   • Remove invalid or bouncing addresses from the list.
   • Avoid very large attachments; use cloud links instead.
   • For Outlook.com accounts, review:
     • Sending limits in Outlook.com (for quota and rate limits).
     • Any NDRs received, and if needed, use Sender Support in Outlook.com when messages are rejected despite valid addresses.

If bulk sending is a core business requirement, consider using a dedicated email marketing or transactional email service that is designed for high-volume sending and provides detailed deliverability diagnostics.

References:

• Can't send or receive email in Outlook.com
• I can't send or receive messages in Outlook