We need a way for the invoice email to be unique for each new invoice we send, as our clients tend to reply to their invoice email when they receive it. Email software (e.g. google mail) groups similar emails together as one conversation based on the subject line being the same. The invoice number is a way to make sure the email subject line is unique each time. Currently the invoice number can only be entered manually into the subject line - therefore we are unable to use the bulk email features in kashflow.