Data Validation Paired with VBA Email Macro

if i'm understanding what you trying to do

    Dim Found As Range
    
    EmailTo = Worksheets("E-mail Sheet").Range("B26")
    
    Set Found = Sheets("Current Clients").Columns("C").Find(What:=Sheets("E-mail Sheet").Range("A26").Value, _
        LookIn:=xlValues, LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:=False)
                                               
    If Not Found Is Nothing Then
        If IsEmpty(Found.Offset(, -2)) = False Then
            If MsgBox(Range("A26").Value & " has already been sent email reminder on " & Found.Offset(, -2).Value & vbNewLine & "Do you want to continue?", vbYesNo) = vbNo Then
                Exit Sub ' stop macro if user selects No
            End If
        End If
    End If

This codes does not check the date sent...only that there is a date
i think you would need another if statement to check the date if you want say within last 10 days or something