External Sharing to Non-Microsoft accounts

StephenRice 

My organization has Office 365 accounts, but we don't use Exchange online so our accounts don't match our emails (email = first.last@domain.com, but Office 365 accounts = first.last@domain.onmicrosoft.com). We have a client sharing files with a user, but it's restricted use, so he can't access them when logged into his Office 365 account, because it doesn't match the email the file was shared with. Does he have to create a whole new Microsoft account based on his email but separate from his existing 

Or is the only option to have the client resend the material to his actual Microsoft account? Since there isn't a functioning email attached to our existing Microsoft accounts, how would he even know?

I tried sharing a file using Excel to another user in my company; her name came up as I typed it & she could see the file under the Shared option, but she never got an email saying the file was shared with her. I tried to see if there was a way to add a secondary email in our Office 365 directory, but I didn't see an option.''

Any suggestions? I'd rather not have to tell a client to do things differently.

Thank you