Additionally, a lot of companies have their emails sent from a fictious address ...
Depending on if your ISP is willing (I do it for my customers), the email server can do a call back to verify that the sending ("from") address is valid. Doesn't check that the email actually came from them, etc. just that it's a deliverable address. It's a 5 minute thing. Downside is that companies that use "from:
no-reply@whatever.com" need to be whitelisted to get through.
I'm with Andrew - get with a email service that provides substantial spam filtering. I have about 6 accounts in use no including the office. All flow 2 the Gmail account. I get on average 500+ spam messages per day (on top of more than that that is filtered at my email servers through Bayesian logic, call back, etc.) that they catch that previous checks did not (they're the last step in the link). They use pooled resources to identify spam, so it's not just your emails that's helping their system to learn.
I deal with upwards of 3-400 non-spam emails per weekday (weekends are much slower thankfully)... without a consolidating mechanism that drastically helpd I wouldn't be able to. Also thankfully, much of it is notification type things that don't require me to reply.
Edit - just checked. Yesterday non-spam = 331 emails. Spam @ gmail = 221, Rejected by MY smtp server (all domains, all account, all attempts (some will try multiple times) = 42,444!!). To those who think spam can be blocked by the ISP easily but won't because of getting paid to pass it to users: 1) Please share how - spam's a major consumer of my resources, and 2) who do I contact to get my checks, mine is over a dozen years late. (Tahiti here I come!)