It would appear that your wife's old phone's email app was set up as a POP3 account, which would appear to have been set to keep the emails on the server after having been downloaded.
Because it was a POP3 email account it does not synchronise with the BT servers (webmail and as such when she deleted the email from her phone, it would not be deleted from the BT servers hence the inordinate number of emails still in her inbox.
If you want to synchronise the email account on her phone with the BTMail (webmail) on the servers you should set up her phone's email app as an IMAP account. That would mean that if she deletes an email on her phone it will be deleted from the BT servers (webmail) and like wise if she sends an email from the phone it will show in her sent folder on the BT servers (webmail).
