Hi Atul,
The recipients maintained under SO28 would be available under the dropdown of FROM in email.
If you have maintained lets say 10 recipients then you will get all the 10s in FROM.
According to your requirement, I see you want to default the From to logged in user's email address. I think to achieve this either you need to enhance the bsp component or find find if there is any BADI available.
Let me search if there is any BADI available, will let you know if I find any.
Regards,
Sanjay