Send mail if today is the birthday.

Sub bdMail()  
Dim OutApp As Object  
Dim OutMail As Object  
Dim cell As Range  
Dim lastRow As Long  
Dim dateCell As Date  

Application.ScreenUpdating = False  
Set OutApp = CreateObject("Outlook.Application")  
OutApp.Session.Logon  
lastRow = Range("A" & Rows.Count).End(xlUp).Row  
On Error GoTo cleanup  
    For Each cell In Range("D2:D" & lastRow)  
    dateCell = cell.Value  
    If Day(dateCell) = Day(Date) And Month(dateCell) = Month(Date) Then  
            Set OutMail = OutApp.CreateItem(0)  
            On Error Resume Next  
            With OutMail  
                .To = cell.Offset(0, 1).Value  
                .Subject = "Happy Birthday"  
                .Body = "Dear " & Cells(cell.Row, "C").Value _  
                        & vbNewLine & vbNewLine & _  
                        "Many Happy Returns of the Day " _  
                        & vbNewLine & vbNewLine _  
                        & vbNewLine & vbNewLine & _  
                        "Cheers," & vbNewLine & _  
                        "Rahul"  
                .send  
            End With  
            On Error GoTo 0  
            Set OutMail = Nothing  
        End If  
    Next cell  
cleanup:  
    Set OutApp = Nothing  
    Application.ScreenUpdating = True  
End Sub