Some mail servers these days are getting ridiculously aggressive about trying to block spam. A perfect example of this is (surprise surprise… not!) Microsoft’s mail hosting services such as Hotmail. If we try to email anyone at a Hotmail address, our mail server immediately gets a rejection before it even sends the content of the message, just based on the IP addresses on our mail server! This is a server that has never sent any sort of unsolicited email, including spam and viruses.
The “reason” given for the block is because it is on a dynamically assigned address. This is a trivial matter, and there is no “rule” that says mail servers must have static IP addresses. Static addresses are responsible for at least as much spam as dynamic ones. Our servers are properly configured: they are not open relays, have RDNS and MX records pointing to them, etc… There is no reason they should be inherently suspect, let alone automatically banned! Now here’s the even more ridiculous part: if we relay our email through our ISP’s mail servers as suggested by Microsoft et al (which we prefer not to for other reasons anyway, such as lack of delivery confirmation), our emails still bounce anyway. No doubt this is because ISPs relay any and all mail for their customers, including spam and viruses, making them likely sources for spam.
So to put it short: We cannot email anyone who uses Hotmail, and this is primarily the fault of Microsoft who runs it.
If you’d like a recommendation for a new email service, try GMail. While they have had similar problems in the past, and still have some pretty big issues (such as not allowing disabling spam filters or supporting IMAP), they also support IM from within the web interface and it is much more reliable than Hotmail! Not to mention that you get virtually an unlimited quota for email space.
-Luke