How about this one then?
https://www.sendinc.com/
I think there is a standard for level of encryption, so I would research it.
Email hosts usually have a limit to file size that can be sent. Some of the files we send and receive for implant planning etc.,
can easily exceed 500MB+ in zipped format. An implant library for my planning software is 5GB compressed for example.
I know there are some encrypted FTP services available. You definitely want to find a service that allows you to keep all of the files
together rather than sending a string of emails and trying to keep track of what goes with which patient case.