Aggressive Spam Filtering May Be To Blame

I have occasionally had the same problem. If if your email goes through to some recipients but not to others, the problem is very unlikely to be in your own email client or computer.

Some intended recipients only permit mail to go through to their clients from specifically allowed senders -- these paranoiacs have brought it on themselves.

However, there are these big spam clearinghouses that many ISPs employ fairly aggressively to prevent the delivery of email from flagged addresses. The most prominent of these is called "The Spamhaus Project," and they maintain an enormous "Blocklist" of IP addresses. Some of these are whole class C networks, and in some cases where ISPs have been extensively exploited by spammers, even entire class B networks. If your address is included on one of these lists, or is passing through an SMPT server that is on one of these lists, your mail will not go through to recipients who receive their emails through ISPs that employ the lists.

Note that you can get on these lists even though you never sent a piece of spam in your life. The SMPT server you use to send your mail is used by others, and one or more of those other users may be the offenders. Your inclusion on the blocklist is just collateral damage.

You can go to their website at http://www.spamhaus.org and check to see if your IP address is blocklisted. Note that the IP address in this case is unlikely to be the one of your individual computer, particularly if it is on a home network behind a router. It is likelier to be the IP address of the SMPT server you use to send mail, which you can obtain from the FULL headers in your email. Your ISP's technical support staff may be able to assist you if you are unable to determine this on your own.

I have found MSN/Hotmail to be a particularly aggressive employer of this spam blocking service.

If you find that you are blocklisted, you can petition to have your address removed from the blocklist. They're pretty good about it, but it takes about a day. Again, your ISP's technical support may be able to help.

The only problem with this theory is that you usually get notified in your client that your email has been blocked. This takes the form of a return post that indicates the specific reason for the non-receipt. This would NOT be the case if the block consisted of a filter in the recipient's email client. There is nothing you can do about that but talk to the recipient -- the intended recipient has to fix it.