So Office 365 doesn’t support Scan to Email without having a device that can take advantage of credentials when sending e-mail, and that also requires a licensed mailboxuser in Office 365.
Solution 1: Create a new user for the purpose. Bills you an extra license.
Solution 2: Reuse a Office 365 credentials you allready pay for, but Scan to Email is for everyone and its not right that one of your employees should be the sender for everyone. Especially if you allow Scan to External Email.
Solution 3: Allmost every ISP has a SMTP Service provided for their customer, and this also support devicer without the ability to use credentials.
Example in Norway is Telenor:
Dynamic IP Customers: smtp.online.no
Static IP Customers: virusvask.telenor.net