You really need to setup SPF for your outbound mail servers. If you are getting spam spoofing your domain, certainly other are, including your customers.
Start with something simple like
apcmmedia.com IN TXT "v=spf1 ip4:1.2.3.4 ~all"
replace 1.2.3.4 with the public IP address of your outbound mail server. If you have more than one, just add more ip4:x.x.x.x items to the record. The ~all means you are not sure this is a complete list. You may need to talk to others at your company to make sure Sales or Marketing haven't outsourced something.
Once you have an SPF record published, you can use DMARC to find out who is sending using your domain. The big ISPs (google, yahoo, microsoft, etc) have deployed DMARC reporting. Start with this DNS record (wrapped for clarity):
_dmarc.apcmmedia.com IN TXT
"v=DMARC1;p=none;rua=mailto:Reports@apcmmedia.com;
ruf=mailto:ForensicReports@apcmmedia.com;rf=afrf;pct=100
p= The DMARC policy, in this case the recipient mail system shoud not do anything special. you can also specify quarantine or reject.
rau= send summary reports ~1x/day to this e-mail address. These are in XML format.
raf= send forensic reports - copies of actual e-mails that failed SPF back to this address
rf = report format. Default if afrf
pct = % of e-mails to apply policy to
I recommend using the free DMARCian.com site to parse the RAU reports. If you sign up, they give you an additional e-mail addess @dmarcian.com to add to the RAU field. I'm using dmarcian and it's reporting on 2-3 million e-mails per day for me.
You also have a spam reputation problem. Both ReputationAuthority and Senderbase show a lot of spam from one of your hosts:
svr02.apcmmedia.com / 69.167.182.82 hosted at LiquidWeb
http://www.reputationauthority.org/domain_lookup.php?ip=apcmmedia.com&Submit.x=22&Submit.y=11
http://www.senderbase.org/senderbase_queries/detaildomain?search_string=apcmmedia.com
Good Luck