well, how could you tell ?
it might be just your local internet connection that's taken a small break
as already mentioned (not that I want to bore anybody...)
a spam filter at the destination adress is
only one part of filtering
internet service providers subscribe to so called 'blacklists' and don't route any traffic from a system that appears on such a list. If one of your provider's system happens to be listed or uses other (gateway) servers that are, then your mail is simply trashed on the way.
These configurations (may) change several times a day... and yet spam is at least 75% of all mail traffic, still
cheers, tom