Please note The default “From” address should identify the account / mail box that WPO365 will always use to submit the email message to Microsoft Graph. Without the premium addon MAIL, emails sent from WordPress will therefore always be sent from this account. With the premium addon MAIL installed and activated, however, you can override the “from” address e.g. by specifying a different “from” header that is compatible with – for example – Contact Form 7. But even then, messages will always be submitted to Microsoft Graph by the default “From” address. Therefore the default “From” address must have been granted delegated permissions to send from the “other” account.

Also note Currently it is not supported (by Microsoft) to send emails from an alias when sending emails using Microsoft Graph. This may change in the near future, since currently sending from an alias is previewed for the various Outlook clients.