First, ‘/QTCPTMM/LOCKBOX’ isn’t a file; it’s a directory. Not sure if that matters.
Second, BRMS is a product, so the vendor (IBM) probably needs to be asked about this.
Third, for us, we’ll need to know the general content of your /QTCPTMM/LOCKBOX directory, the overall configuration of your save request and the status of TCP/IP and SMTP in particular when the save runs. There might be other details needed later such as the number of SMTP e-mail users and an estimate of SMTP e-mail volume held in the system.