When you have the Leads module enabled, it functions like an email inbox when someone emails an Inbox (defined in Admin > Emails - Mailboxes (inbound email)) or fills out and submits a form that emails an Inbox. In Admin > Tickets - Preferences, when you have Create Tickets from Leads (if valid) enabled, before creating the ticket from the lead it will create a new customer if it can't find an existing one in the system. This is because all tickets must be attached to customers.
If you do not want to automatically create new customers, you can do one of two things.
- Disable the aforementioned Create Tickets from Leads (if valid) setting. This will leave any emails in the Leads module.
- Require some customer fields in Admin > Customers - Preferences that emailed leads will not fulfill.
In both cases you can then assign and convert a lead to the correct customer.