Everyone knows how big a problem receiving spam has become for many companies, but another growing problem is the amount of spam companies are unwittingly sending out.

E-mail security vendor BorderWare says it has monitored outbound e-mail of more than 8,000 customers in 65 countries and discovered that on average, 80% of the mail sent out from the customer’s domain is spam.

The reason: Spammers are spoofing these domains so that it appears messages are coming from legitimate sources, but they are actually unwanted e-mails. Also, enterprise PCs are falling victim to botnets, malware that takes over computers and turns them into spam servers without users knowing it.

This is becoming a problem for companies because they risk ending up with bad e-mail sending reputations. Blacklists, reputation services and other sender-rating methods are likely to block mail sent from these domains from reaching their recipients, according to BorderWare officials, because many of these services look only at the volumes of mail sent from a domain and assume high volumes of messages mean spam.

The BorderWare Security Network, or BSN, is a reputation service that does real-time content inspection of e-mail messages to get a detailed view, and factors the content of messages into the sender’s rating, officials say.