VBA Code / Place Specific Text In Cell When Email Sent

find a way to place a value of 1 (or a text "email sent: mm/dd/yy") in a specific cell on each row when an email is sent out through some VBA code I currently have. Then when the workbook is opened on another day, the code will look to see if that cell (target) is populated so that it doesn't trigger a second, third (repetitive) email every time the workbook is opened.

The spreadsheet contains rows of many clients, and growing every week. The code needs to be designed to search through each row, along a specific column (lets suppose column R beginning at row 7). When it finds the target rows blank, the code will turn each target cell red, as well as trigger a message box alerting that follow-up action is required.