I want to use the content of a cell in a google sheet to control whether to send an e-mail out or not - email

Question

I have a spreadsheet for recording customer samples. A column [I] is used to calculate when the original entry date is over a certain period which then displays an overdue message using an if statement. I want to trigger a script to check this column daily and if 'overdue' is found to send an email to an address contained in another column [C]. I am also using another column [J] to return a message when an email has been sent. I am using the script below but am returning errors
The script marks all rows in column J with the sent email message
I appear to be collecting a bigger array of data have needed [adding two rows the the bottom]
Anyone any ideas?
//Setup the function
function autoMail () {
var ss = SpreadsheetApp.getActiveSpreadsheet(); //Get the active spreadsheet
var sheet = ss.getSheetByName( 'TS_Requests' ); //Get the active spread
var startRow = 3; // Start of row that going to be process considering row 2 is header
var lastRow = sheet.getLastRow(); // Get the last row of data to be process
var lastColumn = sheet.getLastColumn(); // Get the last column of data to be process
var range = sheet.getRange(startRow,1,lastRow,lastColumn); //Getting the specific cell to be process
var data = range.getValues(); // Get the values inside the cell
for(var i=0;i<data.length;++i){
var row = data[i];
var email = row[2]; // Grab the email column. C in this case
var customer = row[3]; // Grab data for Email body
var item = row[5]; // Grab data for Email body
var description = row[6]; // Grab data for Email body
var status = row[8]; // Grab the column containing the value needed to be checked.
var EmailSent = row[9]
if (EmailSent != 'Email_Sent') { //Prevents e-mail being sent in duplicate
if(valstatus ='Overdue'){ // Check the values, if it's what's needed, do something
MailApp.sendEmail(email, 'Your sample request is overdue', customer + ' - ' + item + ' - ' + description); // if condition is met, send email
sheet.getRange(startRow + i, 10).setValue('Email_Sent');
}
}
}
}