H-sphere allows to create SIMPLE 5 DIGITS password for mail boxes.
To restrict password policy change the following file with validation rules:
Also you can use Customization Guide where basic customization ruler are explained.
Also please take a look into the YAFV Validation section in H-sphere developer guide on validation rules, that document still valid for 3.6.3 version of H-sphere.
s_hsphere.yafv may be overridden like any other template, thus validation rules for passwords may be changed(see vAllowedPass regexp).