You will have realised by now that the security level in Office 365, including SharePoint and OneDrive, does not let you share out of the organization easily.
However, as you will also have realised by now, Office 365 is more flexible and often there is a way to do what you want if you can figure it out.
The system needs to verify that the email address accessing the files is meant to have access. External contacts emails are not registered in our Organization and putting them in a To box in email doesn’t register them.
The following steps will add external email addresses into the system as guests.