What you need is to iterate through the AB column and check for the "N" entries (assuming it's just a string), then send the email and then, to avoid the recurrence you can change the N to some other value. Using app scripts it would look something like this:
function myFunction() {
var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
// Will return a 2D array with the entire's sheet values
var data = sheet.getDataRange().getValues();
for (var i = 0; i < data.length; i++){
if (data[i][0] == "N"){
// Sends the email, The name section is the name people who receive the email would see ass the "from" id.
// If there is no email in the cell this will return an error, so you can add a try/catch here to avoid that.
GmailApp.sendEmail(data[i][1], "Subject", "Message", {name: 'N Finder Script'});
//To avoid repetition, you can edit the N value after sending the email.
sheet.getRange("A" + (i+1)).setValue(""); // This would be "AB" in your case.
sheet.getRange("A" + (i+1)).setValue("S"); // S for email sent, for example.
}
}
}
Some small caveats; I ran this with a small test spreadsheet, which is why I search for the value in the "A" column. When I check data[i][0] I am checking the ith row of the first column, you have to change the 0 to the number that matches the column where you have the "N". The same applies to the emails, I look at them in data[i][1], you would change that 1 to match the column where you have the email list.
Lastly, to make the code run every time an N is added, you can add an onEdit trigger, you can do this by following the instructions in the following link:
https://developers.google.com/apps-script/guides/triggers/installable