Why are e-mails consider Spam e-mails

Would any one care to comment on this:

Our company’s outgoing e-mails are consider to be spam, so they are being block by some of the receivers.

What type of configuration on the Exchange 2K (if in fact it is the Exch)will cause to flag the e-mails as spam e-mails?

Any comments are appreciated.

Regards

Jorge