Unfortunately there is no 100% guaranteed way to ensure Outlook or other email programs will not mark your email as SPAM and place it in the “Junk Email Folder”.  Spammers are getting very hard to deal with these days and they are wasting EVERYONE’S time, and as a result email systems are cracking down and in some cases they just miss the mark and put REAL email in the SPAM folder.  There are a couple things you can do to “play it safe” however. 

First, if the email is regarding something vital, like a potential sale or meeting time, make a quick follow-up phone call to let them know that your did send an email and to be on the lookout for it.  Mention that if they don’t see it in their “Inbox” to also check their SPAM folder.

Secondly, you can put a line on your website’s contact page (or any page that has a contact form) that says something like “be sure to add the email address info@yourwebsite.com to your “Safe Senders” list so that our response does not get marked as SPAM. 

The last thing is to make sure your email’s content uses the following guidelines: Make sure your subject line is clear and that your email does not use any of the typical keywords that a spammer might use (e.g. “HOT DEAL”, “Special Offer”).  Make sure you do not use ‘ALL CAPS’ in your email.  Avoid using rude or offensive language.  Avoid attaching files in mass mailings.  Avoid using highly formatted text with colored headings.  Overall just write content as you would speak normally and this should be good enough. 

http://blog.thinkworx.com/2008/01/16/how-to-stop-your-outgoing-email-being-marked-as-spam/