I seem to not be getting some emails?

[Jason]

I've received a number of questions lately around this same topic so I felt it would be helpful to explain here as a reference.

As you've likely noticed, the sheer number of spam messages has increased significantly over the past several months.  Starting in 2005, we added blacklisting to our servers to dramatically cut down on the amount of incoming messages.  (See this thread for the announcement from 2005).

So since that announcement, we've employed the larger Spam Blacklists to help reduce spam.  The way this works it that there are a lot of organizations which track and list IP addresses of reported mailservers.  Then, our servers go out and retrieve these published lists every so often and block incoming email from the IP addresses that are identified on these lists as spam senders. 

I seem to not be getting some emails?
Using blacklisting greatly cuts down on known offenders, however, it does mean that from time to time, legitimate addresses may be blocked.  For example, (and this is a hypothetical example I'm making up), if your local ISP has 500 users that all send email through the same mailserver and 1 of their clients launches a spam campaign that gets that mailserver listed, that will impact everyone that sends email through them.  That can certainly be problematic, although, most all of these lists offer fairly easy ways to get your IP address de-listed.  Typically speaking, ISP's monitor these lists and get their IP's delisted if there's a legitimate reason to do so.  In other words, just because an IP is listed today, it might not be in a few hours.

Why do I sometimes get email from someone and other times not?
Larger ISP's may have dozens and dozens of different mailservers.  It's possible that I could send an outbound email that goes through a different mailserver each time.  If one of them was blocked, that specific email may not make it out to my intended recipient.

What can I do if I think this is affecting me?
If you have any questions, please send me the email address of the address that isn't making it to you and we'll search through the logs to see what IP that address came in on. That's the only easy way to see why a specific message didn't make it to you. 

If you happen to know their mailserver IP, you can query it on dnsstuff.com to see if it's listed.

http://www.dnsstuff.com

There is a field on that page called "Spam Database Lookup" and if you enter an IP there, it will return results from all the blacklists out there as to whether that particular IP is listed.

Hopefully this helps explain this a bit.

Regards,
Jason