If you want to filter incoming mail using custom rules there are a few ways of doing this, depending on your exact requirements.
Apart from the User message rules, these methods apply to all messages arriving through the specified method (POP3 for download rules, SMTP for SMTP rules, and all messages (depending on settings) for Spam filter/content filter scripts and Lua scripting). You could make the rules/script only apply to a certain user or users, but that would need to be explicitly defined within the rule/script.
The Download/SMTP rules and user message rules are generally easier to configure than the scripts, as they use drop-downs etc to allow you to configure the rules (similar to 'inbox rules' or filters in many email clients), but are more limited in functionality. The scripts are much more powerful, but involve creating a script in a simple programming language so may not be suitable if you are not comfortable with that.
We are able to produce content filter/Lua scripts for you if you wish, but these will be chargeable, starting from £50+VAT. You will need to contact us with exact requirements for us to give you an individual quotation.