Reply to ticket from an external email application

Reply to ticket from an external email application

We use quickbooks to send an email with invoice and a link. The only way we know how to get into the ticketing system is through by bcc our ticketing email and then merge the ticket. The emails to our ticketing system is also being collected in Outlook, I found out that if I reply to the ticket directly via outlook and bcc our ticketing email it will correctly attach the reply to the ticket. 

The question is how do we get an email sent via QB email to automatically attach to the ticket. We tried using the ticket number + subject line and we ended up with another ticket instead.