Why did my custom tab/ribbon disappear after sending it through email?

Yesterday, I wrote a macro and installed a custom tab with a button in that tab to run the macro. I emailed the Excel file to my boss. This morning when she opened the attachment, the custom tab/ribbon that I added was not there.

What happened? How can I ensure that it does not disappear when somebody else opens the workbook?

Could it be that the person on the receiving computer must have certain Object Libraries enabled?