How many devices do you use your email account on? If more than one have you checked them?
How are you accessing your email. Is it with a web browser or an email client or email app?
If it is with a web browser, if there is a number showing it shows how many unread emails are in your Inbox. You can bring all the unread emails to the top of your list by clicking on the upside down triangle at the right hand side of your list of emails.
If the number is showing only when you hover over the Inbox this shows the total number of emails in the Inbox and how many are unread.
This issue has previously been reported after BTMail updates so if after checking that there are no unread emails in your Inbox or indeed no emails in your Inbox, it is most likely an issue after the recent update (there were other issues reported) with the numbering system rather than an email having "fallen of the system".
