I want send email to external user- not in sharepoint i.e to firstname.lastname@example.org through hosted app workflow.I found a solution to create Send email activity in SP2010 and call it in hosted app workflow but, how to create SP2010 workflow in hosted app(SP2013)?also not sure how do I create 2010 workflow in visual studio 2013. I have also tried to do setting of allow "External sharing" to "Allow both external users who accept sharing invitations and anonymous guest links"but, still not working.Thank you in advance.
As far as I know, there is no way to deploy SP2010 workflow in SharePoint apps. However, you can use built-in Email Activity in SP2013 workflow. It doesn't matter whether you send email to an internal or to external user.
Please, take a look at the following article: Step-by-step: Sending email messages from a workflow in an app for SharePoint 2013
UPDATE 1. It seems you stumbled into by-design behaviour of 2013 WFs. Please, have a look at the following article: SharePoint 2013 - Unable to send emails to external email addresses (SharePoint 2013 workflows). Probably, the solution N1 in the article can work for you.