SpamFilter will log that error when there is a timeout occurring when making a DNS query again your DNS server(s). Depending on the daily email traffic, there could be a few hundreds of these events per day, which is usually not a high number. For each email that SpamFilter receives, there can sometimes be as many as 5-20 DNS lookups being made for various filtering tests, so it's very possible that some may end up in timeouts.

There's a setting in the SpamFilter.ini file that controls the timeout value - we do not recommend setting values higher than 5000 milliseconds to avoid affecting performance: