Many emails are not received when sent using Mail Merge Outlook

Question

When approximately 350 email are sent using Mail Merge, many email are not received by the recipients. How can this be solved?

Answer 1

Hi @Pensioen Werkgroep

I understand that you are experiencing an issue when sending bulk emails via Mail Merge, where a significant number of messages are not being received by recipients.

To help us investigate this issue more effectively, could you please provide a bit more information on the following: 

Did you receive any NDR (Non‑Delivery Report) or bounce‑back emails when this issue occurred? 

Could you please advise how frequently you send emails and approximately how many emails are sent per batch? 

Based on my research, this issue may be related to email sending limit mechanisms. When a large number of emails are sent at once (as your description, around 350 emails), the email system may apply sending limits or spam‑control measures. As a result, some messages may be delayed, throttled, or blocked by the mail server. To avoid this issue, I recommend sending emails in smaller batches, adding delays between sends, or using a dedicated bulk email service for high‑volume mailings. 

For further information, you can refer via: Exchange Online limits - Service Descriptions | Microsoft Learn 

Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

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Answer 2

For large Mail Merge email runs, there are three common causes in the provided context and corresponding mitigations:

1. Using New Outlook for Windows (Mail Merge only partially supported)
   If Mail Merge is run from Word and New Outlook for Windows is the mail client, Word’s MAPI handoff to Outlook is only partially supported. The merge appears to run, but messages are not actually sent and do not appear in Sent Items. Mitigation:
   • Switch back to Classic Outlook for Windows and run the Mail Merge again. Classic Outlook fully supports the MAPI integration that Word uses for Mail Merge to email.
   • Ensure Outlook is set as the default mail app and that the same Microsoft 365 account is signed in to Word, Excel, and Outlook.
2. Messages not being delivered or generating NDRs (delivery failures)
   When sending many messages (such as ~350), some recipients may not receive them due to:
   • Recipient server issues (timeouts, remote server unavailable).
   • Recipient mailbox full.
   • Spam/blacklist filters on the recipient side.
   • Invalid recipient addresses.
   Mitigation:
   • Review any Non‑Delivery Reports (NDRs) returned for the mail‑merge batch. The NDR text usually states whether the address is invalid, mailbox is full, server is down, or the message is blocked as spam.
   • If NDRs mention that the message is blocked or listed on a blacklist, contact the recipient’s email provider support to request unblocking of the sending account or domain.
   • Verify all recipient addresses in the data source are valid and current.
3. Outlook.com / sending‑limit or spam‑filter behavior (if using Outlook.com accounts)
   If the sending account is an Outlook.com mailbox, additional constraints apply:
   • New or low‑reputation Outlook.com accounts have low sending quotas that increase over time.
   • Large batches can trigger spam filters or rate limits.
   Mitigation:
   • Check for NDRs and their reasons (invalid address, mailbox full, server issues, spam filters).
   • If the account is new, build reputation gradually and respect sending limits described in Outlook.com sending‑limits guidance.
   • Ask recipients (or their admins) to add the sending address/domain to their safe‑sender list if messages are being filtered as spam.

In practice, to solve the issue for a 350‑recipient Mail Merge:

1. Run the merge using Classic Outlook (not New Outlook) as the default mail client.
2. Confirm that messages appear in Outlook’s Sent Items after the merge.
3. Monitor and analyze any NDRs for failed recipients and correct addresses, mailbox issues, or spam‑blocking as indicated.
4. If using Outlook.com as the sending mailbox, respect sending limits and, if needed, split the send into smaller batches.

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References:

• Mail Merge Emails not sent - Microsoft Q&A
• Merge to Email problem - please help - Microsoft Q&A
• Can't send or receive email in Outlook.com
• Sent email in Outlook.com comes back "delivery failed"