Hi Satchi,
This error is telling you that you cannot send email on behalf of another person or account.
The "You" is the logged in account trying to send an email.
The "Selected user" is most likely what is being put in the From field of the email being sent.
So, if you are logged in as John and you're trying to send email from Sam you will fail without the right to send email on the behalf of another, which Microsoft calls the send-as privilege here.
This blog post may help: http://www.reconnected.be/archive/2011/crm-2011-user-does-not-have-send-as-privilege