forget yesterdays comment - i was caught up in my own .SendObject problem and wasn't thinking straight.
does this do the job:
private sub myBox_BeforeUpdate(Cancel as integer)
dim recs as DAO.recordset
set recs = Me.Recordset
recs.Findfirst "theFieldName = " & myBox.value
if recs.NoMatch then
msgbox "Potential Error is trapped"
Cancel = True