The dangers of convenience
Once again the lack of proper procedures are the linchpin through which scum can ply their trade. Taking account details from an email means that you do not have a proper client db with the reliable data already inserted.
Which in turn means that your payment transfer system is probably a mess (no check on account number for an existing supplier ?) and errors like this will slip through unnoticed until you get an invoice unpaid letter and start wondering why - which is never the right reason to check your accounting procedures, but better late than never.
This is the kind of pain that will prompt more attention to detail. It is unfortunately a costly lesson, but there is a portion of the population that only learn by costly lessons (backups, anyone ?).