CustomAction is working. Thanks again.
Yes I will install addin on each recipient's machine.

Now I have another problem that these voting buttons are not appearing
when the mail is opened to read. So how can I make it appear when the
mail Item containing it is opened. Is it so that I am missing
something?

Another point is when that mail is opened from my Sent items folder
the voting buttons are appearing and works as expected. But I also
want that when the voting button containing mail item is opened from
Sent Items folder it should not display the voting buttons(i.e. I
don't want them to work there). Voting buttons should appear only when
they are opened to read from the Inbox.

-Mohit