Perhaps your ISP is applying its own spam filters/blockers, etc. without your knowledge or consent. Some do, to cut down on useless email traffic or some of their customers employing spam-like techniques that result in complaints being sent to them.

There is no good reason you can't send email to yourself, especially to the same eddress you are emailing from. That is the first diagnostic technique you try to see if your email is working.

Whenever one of your emails does come through, try this: right click on that email in the inbox and select "Add sender to Address Book". Then use THAT entry to send emails to yourself. It sounds redundant, but for some reason it works. (this is assuming your are using Outlook or Outlook Express for an email client).