On Mac
Create a directory and put the file you want to send in it.

3. Use Disk Utility tool to create a New Image

4. When presented with this dialog, select Image Format as "compressed" and set Encryption to "256-bit AES encryption (more secure, but slower)":

5. Go to https://lastpass.com/generatepassword.php and create a password using the exact following settings:

6. Click save and wait til it's finished:

7. Send the .dmg file to Boardingware team via Intercom or Email.

8. Ask Boardingware team to give you a call back and dictate the password by phone.
On Windows
Use 7zip and perform steps similar to the above.