My WP7 application requirement is to send the email with an attachment and use device default SMTP settings.
I have tried EmailComposeTask class but it doesn't have any member for attachment.
The other idea, i am thinking is to upload the file on server and then send the email from that server.
What you think if there is no way to send the email with an attachment using WP7 API?
Your observation is correct, at this point in time EmailComposeTask doesn't support attachments and this is the only facility to send email programatically from the device at the moment.
As you note, you can communicate with a server which can perform this task on behalf of your app.