น่าจะประมาณนี้ครับ
Private Sub CmbDeleteFromRejected_Click()
Dim sql As String
sql = "UPDATE Personal_Contacts SET Personal_Contacts.PC_EMAIL = Null WHERE (((Personal_Contacts.PC_EMAIL) In (select EMAIL from [Rejected_Emails])));"
DoCmd.SetWarnings False
DoCmd.RunSQL sql
DoCmd.SetWarnings True
End Sub