I also use a separate email alias for each company I deal with. I run my own Postfix servers (primary + backup), but I've now created a web UI with a view to making this usable by others. The URL is in my profile (not sure if posting it here would be considered spam). It's still early days, but seems to work fine for my own emails. If you'd like to use it I'll be happy to create a (free) account for you.