To send mail via smtp.office365.com, a correct "from" address is required. If you don't set "from" in the pipleine step, the plugin should use the admin email. The first time the plugin is used, this address is fixed and can't be changed in the UI.
I believe the problem is with this code in ExtendedEmailPublisherDescriptor:
Once an address has been recorded it can never change, even if you change the admin address. In my case it was the "nobody" address.
The address is recorded in Jenkins home directory, file .jenkins/hudson.plugins.emailext.ExtendedEmailPublisher.xml. A viable workaround is to edit the file and correct the address element. Example:
Then restart Jenkins.