First something general: You can see in the source code of the email why it was tagged as spam. Most spam filters set some information in there. it will look something like this:
X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on server.domain.com
X-Spam-Status: No, score=-0.8 required=5.0 tests=BAYES_00,DC_IMAGE_SPAM_HTML,
With some googling you will find out what everything means.
And now regarding your question: It could be that your domain name has some string in it that is related to porn. A friend of mine had once a similar problem. The administrator within his company decided that "boys" somewhere in a domain name is not work related and potential porn. So he started to block it. The problem was now that my friend had to be in contact with "boyscouts" (in domain names often written together like this) because they were sending fund-raising requests to him.
So my recommendation is to check the email source code. Maybe your domain name is not your only problem. So if you can fix the other things then you can still send emails with your domain name in it.