Office 365 email shows outlook_[long series of letters and numbers]@outlook.com when sending

After you have added an email account to Outlook for Windows (Outlook 2016, 2013, 2010 versions), you may experience the following:

• The name of your email account is shown as outlook_[long series of letters and numbers]@outlook.com in the folder pane and when composing emails with Outlook for Windows.
• Recipients also receive the email from the sender in the format outlook_[long series of letters and numbers]@outlook.com.
• When recipients reply to the email, the email is returned and you may receive the message 'Undeliverable message when you send internet mail in Outlook'.

This problem can occur when you register a third-party email address (for example an email address that is not Outlook.com or Hotmail.com) for your Microsoft account and do not use Outlook.com to send and receive email.

If you normally use an Outlook.com account for email, this problem can occur if there are several aliases on the account and the third-party email address is the primary alias. See the ALTERNATIVE SOLUTION section below.

TEMPORARY SOLUTION

• Set up your account manually as an IMAP or POP account in Outlook.
• Close Outlook
• Use the Windows key + R, then type "outlook /profiles" (leave out the quotation marks) in the Run window and press "OK" Note: there is a space between Outlook and the forward slash (/)
• In the 'Choose Profile' box that opens, select Options > New
• Type a new, unique profile name and select OK
• Choose Advanced options > Let me set up my account manually.
• Enter your account password and then choose Connect.

If the installation fails, select Change Account Settings. Update the account details with the details you received from your email provider or from the reference topic about POP and IMAP server names. Choose Connect and then select Done.

In the Choose Profile window, select the option Set as default profile and select OK.

ALTERNATIVE SOLUTION

Switch your alias to an Outlook.com or Hotmail.com account.

Note: If you do not have another alias, select the link on the page to create one. Choose Email and add a new alias, then choose the option Create a new email address and add it as an alias.

Go to https://account.live.com/names/Manage and sign in to your Outlook.com account.

Set your Outlook.com email as the primary alias.

Updated on: 09/09/2026