Email to be sent when a content is entered in a cell in Excel

There's no straightforward way in Excel to do this. For example, there's no "Alert me on cell changes" feature.  There are a few ways that I think you could do it.

Maybe the email part isn't important. You could create a workbook for yourself and use Power Query to gather all the data from the expense reports that you can review periodically. Assuming you know where the employees are saving their reports, and you have a way to detect when you're missing some data, you would not need to look through every report. Power Query would allow you to filter down to just the out-of-pocket expenses, assuming the format of the reports is consistent.