If a customer sends us an email, will the body of that email show up as a ticket comment?
You'll want to give your customers your inbound email alias address (Admin > Ticket Preferences). If you want tickets to be automatically created from leads, make sure that that the "Create Tickets from Leads" setting is checked too.
Then, when customers send an email to that address, the lead will show up in your Leads section.
The email subject will be the Ticket Subject. The email body will be the Ticket description.
