Sending, receiving, and organizing email in Outlook on the web for business
If a message cannot be delivered to another email address, the most common fixes are:
- Check the recipient address for typos.
- Delete the saved address from the To field and remove it from Auto-Complete.
- Type the full recipient address again manually, then send the message again.
- If the mailbox is reported as Mailbox not found, Invalid mailbox, or User unknown, the address does not exist or has changed. Confirm the correct address with the recipient.
- If the error says Mailbox unavailable, wait a short time and try again.
- If the message is too large, upload large attachments to OneDrive and send a link instead.
- If sending from Outlook or another mail app fails, try sending from Outlook.com.
If the failure message came from ******@outlook.com, postmaster@microsoft.com, or service.microsoft.com, review the bounce message and look for the Diagnostic information for administrators section. That section usually shows why delivery failed.