If you have a version of VPOP3 prior to v7.7, and you have the VPOP3 Connection Schedule configured for more frequent connections than every 5 minutes, then you may see in the Settings -> Spamfilter page, the text "HTTP Error - HTTP/1.1 429 Too Many Requests"
This is nothing to worry about. The update server is throttling requests for spamfilter update checks to once every 5 minutes per IP address. This is because the server was being heavily loaded by very frequent requests which were not doing anything other than forcing licence queries on our backend database. VPOP3 v7.7 will now throttle update checks itself, so you won't see the errors in that version or later.
Note that if there is a problem getting spamfilter updates, VPOP3 will send an email to the VPOP3 administrator telling them of the problem. You will not get such a message because of these 'Too many requests' warnings, because VPOP3 is successfully checking for updates every 5 minutes or so, so it knows that there is not a persistent problem.