CANNOT SEND EMAILS

For Outlook.com/Hotmail, the most likely causes for undeliverable returns when sending to a large group are sending limits, spam filtering, or recipient-side blocking.

Try these steps in order:

1. Reduce the number of recipients per message.
   • Sending to 104 members at once can trigger delivery failures or temporary restrictions.
   • Send to smaller batches instead of one large message.
2. Avoid putting all recipients in BCC.
   • Outlook.com sender guidance specifically says not to put all recipients in BCC.
3. Do not send too many emails at once.
   • Outlook.com can apply a temporary low sending quota to newer accounts until credibility is established.
   • If the account was created recently, check the Outlook.com sending limits information.
4. Add recipients to Contacts.
   • Outlook.com recommends making sure the people being emailed are added to Contacts.
5. Ask recipients to add the sending address to their Safe senders list.
   • This can help if messages are being blocked or treated as spam.
6. Check whether the message is being blocked as spam.
   • If the non-delivery report includes terms like blocked, listed in, spamcop, dynablock, blackhole, or spamhaus, the recipient server is blocking the message as suspected spam.
   • In that case, contact the support team for the recipient’s email provider and request that the account or domain be unblocked.
7. If the message is returned as undeliverable, verify the recipient addresses and reduce recipients further.
   • Microsoft’s guidance for undeliverable errors is to confirm the addresses are valid and reduce the number of recipients.
   • If sending from an email app, try sending from Outlook.com on the web instead.
8. Check for common Outlook.com sending issues:
   • Remove any email signature and try again.
   • Make sure the message does not include attachments larger than 25 MB.
   • If a non-delivery receipt continues after confirming the addresses are correct, use Sender Support in Outlook.com.

If the account is specifically showing a prompt to upgrade even though Microsoft 365 was already purchased, the supported guidance in the available material is to check whether the account can sign in normally and whether the account is temporarily blocked due to unusual sign-in activity. Updating the password if it was changed recently can also help.

• Can't send or receive email in Outlook.com | Microsoft Support
• Sent email in Outlook.com comes back "delivery failed" | Microsoft Support
• Sender Support in Outlook.com | Microsoft Support