Another possibility, you do not have permission to use mail profile?
according to http://msdn.microsoft.com/en-us/library/ms190307.aspx
Execute permissions for sp_send_dbmail default to all members of the DatabaseMailUser database role in the msdb database. However, when the user sending the message does not have permission to use the profile for the request, sp_send_dbmail returns an error and does not send the message.
To grant permission
EXECUTE msdb.dbo.sysmail_add_principalprofile_sp
@profile_name = 'Email_Profile',
@principal_name = 'UserORrole'