Auto Send Email based on Cell Content

I am trying to get an email to automatically be produced and sent if Cell in Column U contains the words "Send Reminder"

I have attached the spreadsheet and the code I have been working on [from Ron Bruin site]for the email which I cannot get working

Also not convinced it would send automatically when the cell changes to contain required phrase

I'd be really grateful for some help with explaining what I have done wrong

Sub email()
Dim lRow As Integer
Dim i As Integer
Dim toDate As Date
Dim toList As String
Dim eSubject As String
Dim eBody As String
 
With Application
    .ScreenUpdating = False
    .EnableEvents = False
    .DisplayAlerts = False
End With
Sheets("Sheet1").Select
lRow = Cells(Rows.Count, 12).End(xlUp).row '12 = email address from Column L
For i = 2 To lRow
  If Left(Cells(i, 21), 12) = "Send Reminder" Then '21 = From Column U, 12 = email address from Column L
     Set OutApp = CreateObject("Outlook.Application")
     Set OutMail = OutApp.CreateItem(0)
        toList = Cells(i, 12)    'gets the recipient email address from Column L
        eSubject = "Mower Service is due on" & Cells(i, 16) ' Next Service Date from Column P
        eBody = "Dear " & Cells(i, 3) & " " & Cells(i, 5) & vbCrLf & vbCrLf & "Your " & Cells(i, 17) & "is due for its next service." & vbCrLf & vbCrLf & "Please call us on XXXXXXXXXXX or email us to arrange for your machines next service." & vbCrLf & vbCrLf & "The Cost of the service will be " & Cells(i, 19)
       '3 = Title from Column C, 5 = Surname from Column E, 17 = Mower Type from Column Q, 19 = Cost from Column S
        On Error Resume Next
        With OutMail
        .To = toList
        .CC = ""
        .BCC = ""
        .Subject = eSubject
        .Body = eBody
        .BodyFormat = 1
        .Display   ' ********* Creates draft emails, to be removed when final testing complete
        '.Send     '********** To be set when final testing complete
        End With
     On Error GoTo 0
    Set OutMail = Nothing
    Set OutApp = Nothing
Cells(i, 21) = "Mail Sent " & Date + Time 'Marks the row as "email sent in Column U"
End If
Next i
ActiveWorkbook.Save
With Application
    .ScreenUpdating = True
    .EnableEvents = True
    .DisplayAlerts = True
End With
End Sub

Spreadsheet is here: Contacts List.xlsm

Thank you so much for helping