first thing first. do you have an anti-spam. If yes juste make sure your domain is not wite listed. if not bye or rent one and this will fixe your probleme.
Spamers are smart and will seek all sort of way to distribute their junk.
“Spoofing” or imitating your email address is a very common spamming technique. Third-party spam blocking software or a third-party vendor who will do this for you are your best options. It depends on how much money you want to spend or how much time you want to devote someone’s job to this. Controlling it yourself will take almost a full-time employee, outsourcing to a third-party vendor relieves you of the responsibility and some guarantee a 95% reduction of SPAM. It really depends on what your company decides is the best option.
You can find a comparison of different antispam products here: http://www.msexchange.org/articles/Server_Based_Antispam_Comparison.html
Try implementing a mail proxy or spam filter in front of your exchange server. We use a Sophos ES4000 and it works great. It is blocking 40,000 spam and viruses every day. It is definitely something to consider.