Spam filters are very private about why your email passed or failed. They do this to ensure that spammers don't figure out their techniques.

There is typically not one simple change in order to let your email pass by a spam content filters.  Instead, most filters use a scoring system, if you score high enough, they block your email.  An "all caps" word may be one point, repeated words may be another point, and $$ in the subject may be 10 points!   Once you are over a certain score, you email is blocked.  There are 100's of spam filters, each with their own unique settings. 

Things to Avoid

Spam-like words
Free, guarantee, credit card, sex etc.

Red text
Red is a loud color and can be hard to read. It is also a spam tactic that may trip an email filter.

All capital letters
Resist the temptation to use capital letters and over-punctuate. When you use all capital letters, there is no differentiation in your words. This makes them harder to read. It COMES ACROSS LIKE YOU'RE SHOUTING, makes your email look like spam and will dramatically increase the likelihood of your email being filtered.

Incomplete information
Your physical address is required by law. Always include your reply email address and your Web site URL, if you have one. Depending on your business, you may decide to include your phone number as well.

Excessive punctuation !!!, ???
This is likely to trip email filters especially when used in conjunction with spam-like words and capital letters.

Excessive use of "click here" especially in all capital letters
Make your call-to-action links more specific to avoid filters.

Excessive use of $$, and other symbols
Again, this tactic is likely to trip email filters. Use just one dollar sign for currency and use descriptive words instead of symbols to get your message across.

No "From:" address or noreply@
It looks like spam and will increase the likelihood of your email being filtered.

Misleading (or missing) subject line

Include both a HTML and text version of your email

Words to Avoid in Subject Lines

This does not include any inappropriate subject lines:

• All caps
• dollar amounts
• spammy looking monetary reference
• owe
• indebted
• other
• wipe out
• remove
• get (?:rid| out) of| eradicate)
• (?:owe| debt| obligation)
• (?:alert| response| assistance| proposal| reply| warning| noti(?:ce| fication)| greeting| matter))
• \bsecurity (?:[a-z_,-]+ )*?measures?\b/i
• (?:Jan| Feb| Mar| Apr| May| Jun| Jul| Aug| Sep| Oct| Nov| Dec)\S* \d+% OFF/
• "At No Cost"
• Cheap in Caps
• talks about money bonus!
• low rates
• looks like Fharmacy spams.
• starts with Do you dream,have,want,love, etc.
• starts with Lose
• says will help
• Re: new \d\d\d
• Attempt to obfuscate words in Subject:
• line starts with Buy or Buying
•  starts with dollar amount