Why is the from address drop-down in Outlook 365 empty?

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There could be several reasons for the from address drop-down in Outlook 365 to be empty:

1. The email account is not set up properly: If the email account is not set up properly in Outlook 365, then the from address drop-down will be empty. Make sure that the email account is set up correctly in Outlook 365.

2. The email account is not added to Outlook 365: If the email account is not added to Outlook 365, then the from address drop-down will be empty. Make sure that the email account is added to Outlook 365.

3. The email account is disabled: If the email account is disabled, then the from address drop-down will be empty. Enable the email account to use it in Outlook 365.

4. The user does not have permission to send emails from that address: If the user does not have permission to send emails from that address, then the from address drop-down will be empty. Make sure that the user has permission to send emails from that address.