While not all spam filters are the same - some are more lenient than others, there are ways that you can try to avoid ending up in your contacts junk folder.

Some of the most common reasons that you may end up being marked as spam include:

• Your subject line includes capital letters, exclamation marks, or grammatical errors;
• Your subject contains words related to selling, e.g: "special offer", "limited time", "free";
• You're on a contacts blacklist;
• You have broken links within your email;
• Your email doesn't contain enough text, it's almost all images.

ActivePipe's Chief Technology Officer Mike has some more tips here: 
Stop Your Listings Emails From Landing In The Junk Pile