I understand if we have a sales and a support mailbox you can decide to keep emails received to the sales mailbox as leads (no auto ticket creation) and for the support mailbox we auto create a ticket. However, we would like to have a simple check box that says "if the email address from incoming email is for an existing customer create ticket, if not create lead only".

We always receive emails in our support mailbox for non customers so having to delete from tickets and the new customer record can be an administration burden.