Firmware downgrades are not often possible with newer HP printers. The 4345 may be one that can accept a previous version but I am not sure. Unfortunetly HP only provides links to the most recent firmware updates for thier models. We also do not keep an archive of old firmware for us to share a link either.
If your goal is to grab a new copy of an old firmware you may be out of luck : /
Do you know if the printer behaves this way for everyone or is it for specific users in your enviornment? Perhaps there is an email misconfiguration that can be corrected by taking a look at your email server settings. You could isolate the problem a little more by attempting to email to the individuals from the scanner to see which ones work and which ones do not.
You could also attempt to isolate a little more by attempting to scan to different file formats. Chances are you are default scanning in PDF. Maybe .Tiff or a flat image file would have more luck?