Why would an email service know that conpany.com is a scam address of company.com? If they were spoofing addresses then it may be detected, but if they are using a valid domain with a random letter changed you are out of luck unless you have common sense and pay a lot of attention.

There are many things that can be done to prevent this and likely often are. I've set my mail client to color the subject line of messages differently if the from address is in my addressbook or not. However, I don't know how common it is for mail clients to do things like that by default so there may be many people who don't have any obvious sign that something is wrong.

ETA: I think some slight variations on the domain name could also be filtered out completely, but I'd guess it would be hard to catch all of those that it is difficult to visually detect without also filtering legitimate mail.