To add your own recipient spamfilter function just follow this steps:

1. look at the existing filters. It is very likely that one of them does already
   what you want or something similar so you can copy this one and use parts of
   it.

2. Write your function. It must have the same signature like the others defined
   in qsmtpd/filters/rcpt_filters.c. Here are some things you should know:

   -use getfile to load a config file from the user directory with fallbacks to
    domain directory and global config
   -if the config entry is a single integer (or a boolean) use
    getsetting{,global} instead and store the value in the file "filterconf" in
    the user or domain directory. This function has a little special behavior
    for booleans: if a user does not set a value (or set it to 0) it will
    continue and scan for this setting in domain and global file. If one of the
    three possible positions sets a value to something != 0 this value is taken.
    If you set a boolean to -1 this means "this is not set and don't override
    this from domain/global setting". This means if a user sets "forcessl=0"
    (or no forcessl at all) and the domain sets "forcessl=1" than forcessl will
    be activates. But if the user sets "forcessl=-1" it will be deactivated.
   -getsetting sets errno on exit. I prefer to see this as a sign that the user
    has deactivated the check (so he has to eat it's own dust).
   -the possible return values of the function are documented in
    include/userfilters.h
   -the logmsg parameter is NULL at the beginning of each function
   -if your filter checks the parameter of the "MAIL FROM:" or it's reverse
    lookup don't forget to handle the case of a bounce message
    (xmitstat.mailfrom.len == 0).
   -the logmsg parameter will not be freed so this has to be a constant. If you
    want to log something dynamically or not fitting in the given logmsg use
    log_write{,n} yourself and leave *logmsg alone
3. add your function to qsmtpd/filters/rcpt_filters.c at two points: add it to
   the list at the top of the file (preferably as the last entry) and add it to
   the array at the bottom of the file. Here you should choose a point you find
   useful. It should be good practise to do all offline checks before the first
   online check. All checks that do not make any network traffic themself (not
   counting writing an error message) are offline checks. DNS lookups, asking
   other mailserver and so on are online checks.
4. add your file to OBJS in qsmtpd/filters/Makefile
5. test it
6. if it really works write a bit of documentation and send it to me so I can
   review it and include it if I find it useful.