Remember: It's my free decision to decide from whom I want to accept e-mail. Nobody got the right to send me e-mail. See also the MAPS Realtime Blackhole List for an explanation.
If your e-mail is a question about sendmail, then you should either post it comp.mail.sendmail or send it to sendmail.org as explained in the README file.
If you really need to send me personally an e-mail, send it to:
ca + blocked at endmail dot org
of course you have to replace "at" and "dot" appropriately.
Your site may also be blocked because of a non-RFC-conformant MTA:
MAIL FROM:<>must be accepted. For example, something like this:
220 X1 NT-ESMTP Server SOME.STUPID.HOST (IMail 4.06 10848-9) ... MAIL FROM:<> 501 bogus mail fromis plain wrong. Obviously some people can't read but believe they can implement MTAs.