Sending a Protected Workbook

Hi,

I'm having a problem sending password-protected workbooks.

I have the workbook protected and saved with a password, and I go to Share and email a copy of the workbook as an attachment. My email says the emails were sent, but my colleague never received them. My colleague has received emails with workbooks that have been protected (and with workbooks that have not been protected) before, but it's stopped working.

Is there anything I can do to fix this?

Thank you