avpmilter is a sendmail milter interface to the Kaspersky antivirus daemon.
Avpmilter gets virus scan result and tells sendmail MTA to send reject message upon virus detection. This program is not MTA, nor antivirus by itself.
In comparison with will Kaspersky's kavkeeper it will improve performance of your mail subsystem because:
It will not reinject each message for each recipient back to sendmail MTA ( if sendmail receives, for example, one message for 30 recipients, kavkeeper will initiate 30 requests to antivirus daemon and 30 additional executions of sendmail program. Avpmilter will send only one request to antivirus daemon and will not initiate additional executions of sendmail program).
Avpmilter is small resident miltithreaded program, thus it avoids expensive fork() calls.
Avpmilter almost does not affects your current sendmail configuration. Moreover milter interface allows to run sendmail MTA and filter on different computers. avpmilter requires sendmail 8.11.x or 8.12.x built with milter support (which is the default option for sendmail 8.12). You can read about how to build and configure sendmail with milter interface in libmilter/README in sendmail source tree. Also you can find a good milter api ocumentation in libmilter/docs directory in version 8.12 of sendmail.