ok, i see the problem.
the problem is that the ID column oyu are storing the values in is a numeric column
the implode function:
$WA_fieldValuesStr = "".((isset($_POST["agent_id"]))?implode(", ",$_POST["agent_id"]):"") ."";
is converting the array yo a comma separated list which is a string, the comma selerated list cannot be stored in a numeric column, it must be stored in a text column.
the email alerts may be getting trapped in a spam folder.