Running a web hosting service can be frustrating, especially where email service comes in. A web server with a bunch of shared virtual domains can mean hundreds of mail users passing email through the server throughout the day. On top of that, the server has to deal with 10s of thousands of spam messages per day. When I bother to look at the statistics about 80%-85% of the mail that hits the server is spam and the overall amount seems to grow weekly. Controlling spam takes a huge amount of resources from servers.
Some mail services have started taking extraordinary steps in an attempt to handle the spam issue. Apparently, one of them is Yahoo Mail. Unfortunately, for their users (anyone with one of those free yahoo.com accounts – I have one) their policies mean that email may be delivered only after long delays or sometimes, not at all.
It’s difficult to explain exactly what they are doing, and I only became aware it recently when a couple of my customers started complaining about lost or long delayed email going to yahoo.com through our mail server. I started looking at our mail queue and 
tracing what was happening to email addressed to people at yahoo.com.
Time and again certain messages going to certain addresses were getting delayed error responses from yahoo.com. This has been going on for quite some time as I discovered at this forum thread. A casual Google search finds this is a widespread issue.
What’s really frustrating is that it seems totally capricious. Some yahoo.com messages get delivered immediately. Others get delayed. If I force a particular message that is stuck in our mail queue, it may get delivered, or it may get delayed. But sometimes if I do it a second time it will go through. What kind of a system is this? Repeated attempts to get through to Yahoo support have ended only in canned responses and no resolution.
If email continues to get delayed, eventually it will get deleted from server mail queues. So, unfortunately, yahoo.com mail users may not even know how much email they are missing.
As a result of this, I recommend no one rely on yahoo.com accounts for anything critical. Switch to gmail on hotmail or something else. Given this situation, not ISP or web host can guarantee delivery of yahoo.com email.
I very strongly recommend Google Apps for your domain (https://www.google.com/a/) It’s basically Gmail@yourdomain.com with other services like calendaring thrown in). Mail never seems to get delayed and the spam filter is excellent. POP access is free. All my clients – cityofarcata.org, hcaog.net, humboldtlafco.org – like it, and I’ve really enjoyed using the service for green-wheels.org and my own sites.
Some webmasters and users really don’t like address munging (admin «at» hta.org), but I still do it. After having accounts fill up with spam when i didn’t munge my address, I’ve made sure new addresses don’t go up on any sites unmunged and it’s meant I receive only one or two spam messages a day (hard to believe, I know). And by receive spam messages, I mean that figure includes spam that gets caught, too.
I’m not so sure about that. The system we use has almost no false positives that I have seen. So, I don’t believe I am missing any crucial email. Also, I see maybe 3 spam messages per day that slip through. But that’s way better than 100-200 per day I would probably see without the filter.
It’s true that it’s nearly impossible for spam filters to be perfect. But some are very good. And for the most part, my email works quite well.
I’d suggest that people can’t really rely on any other e-mail, either. There’s just no way to get the spam filters right.