The 3-digit numbers are in fact the password in encrypted form. The "official" way is to log into YAM as a user with manager privileges (usually the first user). This user can change any other user's data. Deleting these 3-digit numbers will effectively clear the password.

This password has nothing to do with the password for your mail accounts. It is just for logging into YAM. Without any password for a single user YAM will start up without any interruption. With multiple users you first have to select the user and then enter the optional password.