HI Siva,
Implement CRM_ORDER_SAVE BADI.
Importing parameter contains GUID that you need to pass in the below FM.
Inside it read CRM_ORDER_READ FM it will give you the value in that field based on the value you acn send mail.
if you need to send mail on some action you need to start SERVER event for that field like on dropdown enter etc.
Let me know if you have more queries.
Thanks & Regards,
Rahul