[jarland]

The reasons are many, but here are a couple:

1. The more spam you successfully receive, the more your email address will be distributed on spam lists, the more spam you will end up receiving as a result. No matter how large their storage array, emails take up space and especially maildir based systems take up inodes for every email. The exponentially increasing flood of spam over time, combined across multiple accounts (many of which receive them at exponentially faster rates than others but would still keep filtering disabled) causes a crisis for sysadmins that can lead to effectively DOS attacking SMTP servers, slow disk I/O, and increased overhead costs at the hands of a third party.

2. Forwarding consistency / IP reputation. People are often confused by inconsistency in what they receive and what is forwarded, but if you take a situation where exponentially increasing spam occurs and then it doesn't all forward, many will wonder if they can trust the forwarding to be consistent. If you do forward it all, your IPs start getting blacklisted at Google, Yahoo, etc. Keep in mind that Google is more likely to view an email as spam for having been forwarded, even when it might have landed in your inbox if sent directly to your Gmail account. If not relevant for you, still relevant for a neighbor that forwards to Google (of which I assure you, you have plenty).

Now it's important to note that not everyone fits into these, but when you're working in an environment that hosts hundreds, thousands, or more users then you really only need a few that do experience this to force a situation where you absolutely have to do this at a global level because you can't predict who it will happen to next, and you can't subject all of your users to the poor decisions of either other customers or third parties who want to send them spam.

There are people out there who can have a catchall with no spam filtering and get almost no spam. Some of them will start getting flooded with spam any day now, at such a rate that they might not even be able to receive legitimate emails due to reasonable connection limits. How do you know if you're that next person? You don't, until you are.

This is why even companies that let you turn off spam filters most likely still have rules that they use to reject email. You can't disable it altogether and run an email service with any reasonable number of customers, for any reasonable amount of time.