The free account allows you to send 100 emails a day.
SendGrid has a sophisticated set of charts for displaying the outcomes of all emails sent through it's API.
If this happens, LeadHootz will update it's database so that future attempts to send a message to this email will be prevented.
Yes, you can use the same SendGrid integration for other accounts your add to LeadHootz that might benefit from email follow-ups. This includes LinkedIn, Facebook and Twitter campaigns.