Cannot link IMAP/SMTP email to M365 account....

Question

When I log into M365 Premium and send email it says from "outlook_1987F8AF81C*****@outlook.com on behalf of".

The emails address it says on behalf of is the one I want to use through M365 but not "on behalf of"

If I create an account in Outlook with my email it lets me use it properly but it is not linked to my M365 account

How can I link my email account to M365 properly?

Thanks

Answer 1

For a consumer Microsoft 365 setup using a non-Microsoft email address, the “on behalf of” behavior is expected and cannot be removed by linking the account differently.

What is happening:

• When a third-party email address is used with a Microsoft 365 consumer subscription, Outlook may use an auto-generated outlook_[long string]@outlook.com identity.
• Sending can appear as outlook_[...]@outlook.com on behalf of <custom address>.
• There is no supported setting in the provided guidance to make that consumer setup send only as the custom address through the Microsoft 365 account identity.

What can be done:

1. Use the custom mailbox as a separate account in Outlook
   • Adding the mailbox directly in Outlook with its own IMAP/SMTP settings is the supported workaround for using that address properly for send/receive.
   • This matches the behavior already described, where the address works properly when added as its own account.
2. If the issue is specifically with an Outlook.com connected account send error, add that address as a secondary alias
   • Go to https://account.live.com/names/Manage
   • Under Account Aliases, select Add Email
   • Choose Add an existing email address as a Microsoft account alias
   • Select Add Alias
   • This is documented as a workaround for connected-account sending issues.
3. In new Outlook, link eligible third-party accounts for premium benefits
   • This only applies if there is a Microsoft 365 subscription on a Microsoft account with the same email identity as the third-party mail account.
   • Path: View tab > View settings or File tab > Account info > Accounts > Your accounts > choose the account > Manage > Link Accounts
   • This links accounts for premium features, but the provided guidance does not state that it removes the “on behalf of” sending behavior.

If the goal is to send only as the custom domain/email address and not show the Outlook-generated address, the provided guidance does not offer a consumer Microsoft 365 method to do that by linking the account differently.

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• Link non-Microsoft email accounts to your existing Microsoft subscription for premium benefits
• Outlook.com email account appears as "outlook_[long series of letters and numbers]@outlook.com" in Outlook for Windows
• Unable to send emails from Outlook.com connected accounts from Outlook app