Why is my mail to this person not getting through?

Legitimate email typically gets erroneously blocked for a couple of reasons. Either the receiving system thinks your email looks too much like spam or the receiving system thinks that you're sending it from an address that is or has been accused of being a spammer. Note that I said thinks. It's the mistakes associated with that thinking that cause legitimate email to be mistaken for spam.

Spam content filters look at your email and assign points for various behaviors that are also associated with email that comes from a spammer. As soon as you collect too many points (where "too many" is up to the receiving system or the individual recipient), your email is flagged as spam. Some of the things to watch for in your email include:

• Sexually explicit terms or phrases, such as "adults only," "over 18," and the like.
• Certain drugs, again typically linked to sexual performance or characteristics.
• SHOUTING. Spam filters will often consider shouting (or alternately SHOUTING or not) as sales copy.
• Fake, inconsistant, or illegal return addresses. Or a "reply-to" address that does not match the "from" address. (If you don't know how to even make that happen, don't worry about it.)
• HTML email. Spam filters consider HTML email as having a higher likelihood of being spam than plain text email.
• Marketing terms. Because so much spam is in fact direct sales marketing, many filters now look for various words and phrases such as "satisfaction guranteed," "free offer," or any of a host of sales wording and give that a higher probability of being spam.

It's important to realize that no one is saying that any of those things in your email is bad or that any one of those things will cause your email to be blocked. The unfortunate reality of the situation is that the more that your email looks like spam, however innocuous, the more likely it is to be treated as spam.

The other common problem is email being blocked because it came from an IP address that's been identified as somehow being related to spammers. If you're not getting your email bounced back to you with some indication, this is both harder to detect and harder to resolve.

A quick test to make sure that any email can get through is to use another provider - a friend's email account on a different service or even a free Hotmail or Yahoo account (although many places filter those addresses as well).

Finally, if you find that your address has been blacklisted or you still can't determine what's going on, it's time to contact your ISP. They're the "owners" of your IP address and are responsible for keeping spammers off of it and keeping it working for you.

And if they're not interested it might be time for a new ISP.