We have lost our OE(8.1F) e-mail Password and consequently are unable to e-Mail(direct method) invoices to clients. All other modules seem to be working.
I tried e-mailing from a back-up and it seems to work.
Is there a particular file from a backup that I can copy to the current data set to correct this issue?

The file that contains the email settings has several other settings in it that could be changed since that backup. So copying the file into the live data would not be advisable.

It would be better to edit the file but I don't think it's appropriate to explain that on here. Have your consultant email me if they don't know which file it is and I can send them instructions.

I believe that I am able to edit a file.
Worst case scenario, I restore original from backup.