Hi,
i have to send invoice to multiple mail, so i change the attribute of the input mail
<input type="email" name="to_email" id="to_email" class="form-control" required="" value="mail1@anywhere.true, mail2@anywhere.false">
to
<input type="email" name="to_email" id="to_email" class="form-control" required="" value="mail1@anywhere.true, mail2@anywhere.false" multiple>
If it can help someone