This usually happens as the result of one of several reasons. Most notably, by not having SMTP authentication enabled in your email client. For example, if you are using the Microsoft Outlook email client, not having SMTP Authentication will cause several points to be added to the spam score. This can cause emails that otherwise would be allowed through to be classified as spam.

Another reason if you are getting caught by the filter is because of the message content in the title and body. If there are any words that are commonly associated as “spammy” terms, this will cause the message to fail if it scores high enough. This link will show you common and popular terms which are of a spammy nature and should be avoided

Why am I getting this error?
To help in the global battle of ever-increasing spam, we have implemented spam detection procedures on all emails prior to leaving our servers. Each email is evaluated and rated according to spam rules to determine how likely they are to be actual spam. Those that fail the test are returned to the original sender with the error message 550 This message was classified as SPAM and may not be delivered (please see the email below). If you are receiving a bounceback with this error, then the email you sent has not passed the outbound filter.

Common words that trigger spam filters
Below is a current list of the most popular words used in spam emails. As such, these words get higher spam points by evaluators. Once an email has too many spam points, it will get flagged as spam, either by outgoing filters or filters on the incoming mail servers. Using one of these phrases once will not trigger an email as spam, but using a phrase repeatedly or many of these phrases together in the same email can cause it to be flagged as spam.

List of most common terms that trigger spam filters

$$$ Free membership Potential earnings
4U Free money Print form signature
Accept credit cards Free offer Print out and fax
Act now! Don’t hesitate! Free preview Produced and sent out
Additional income Free priority mail Profits
Addresses on CD Free quote Promise you …!
All natural Free sample Pure profit
Amazing Free trial Real thing
Apply Online Free website Refinance home
As seen on Full refund Removal instructions
Auto email removal Get It Now Remove in quotes
Avoid bankruptcy Get paid Remove subject
Be amazed Get started now Removes wrinkles
Be your own boss Gift certificate Reply remove subject
Being a member Great offer Requires initial investment
Big bucks Guarantee Reserves the right
Bill 1618 Have you been turned down? Reverses aging
Billing address Join millions of Americans Risk free
Billion dollars Laser printer Round the world
Brand new pager Limited time only S 1618
Bulk email Long distance phone offer Safeguard notice
Buy direct Lose weight spam Satisfaction guaranteed
Buying judgments Lower interest rates Save $
Cable converter Lower monthly payment Save big money
Call free Lowest price Save up to
Call now Luxury car Score with babes
Calling creditors Mail in order form Section 301
Can’t live without Marketing solutions See for yourself
Cancel at any time Mass email Sent in compliance
Cannot be combined with any other offer Meet singles Serious cash
Cash bonus Member stuff Serious only
Cashcashcash Message contains disclaimer Shopping spree
Casino MLM Sign up free today
Cell phone cancer scam Money back Social security number
Cents on the dollar Money making Special promotion
Check or money order Month trial offer Stainless steel
Click below More Internet traffic Stock alert
Click here link Mortgage rates Stock disclaimer statement
Click to remove Multi level marketing Stock pick
Click to remove mailto Name brand Stop snoring
Compare rates New customers only Strong buy
Compete for your business New domain extensions Stuff on sale
Confidentially on all orders Nigerian Subject to credit
Congratulations No age restrictions Supplies are limited
Consolidate debt and credit No catch Take action now
Copy accurately No claim forms Talks about hidden charges
Copy DVDs No cost Talks about prizes
Credit bureaus No credit check Tells you it’s an ad
Credit card offers No disappointment Terms and conditions
Cures baldness No experience The best rates
Dear email No fees The following form
Dear friend No gimmick They keep your money — no refund!
Dear somebody No inventory They’re just giving it away
Different reply to No investment This isn’t junk
Dig up dirt on friends No medical exams This isn’t spam
Direct email No middleman University diplomas
Direct marketing No obligation Unlimited
Discusses search engine listings No purchase necessary Unsecured credit/debt
Do it today No questions asked Urgent
Don’t delete No selling US dollars
Drastically reduced No strings attached Vacation offers
Earn per week Not intended Viagra and other drugs
Easy terms Off shore Wants credit card
Eliminate bad credit Offer expires We hate spam
Email harvest Offers coupon We honor all
Email marketing Offers extra cash Weekend getaway
Expect to earn Offers free (often stolen) passwords What are you waiting for?
Fantastic deal Once in lifetime While supplies last
Fast Viagra delivery One hundred percent free While you sleep
Financial freedom One hundred percent guaranteed Who really wins?
Find out anything One time mailing Why pay more?
For free Online biz opportunity Will not believe your eyes
For instant access Online pharmacy Winner
For just $ (some amt) Only $ Winning
Free access Opportunity Work at home
Free cell phone Opt in You are a winner
Free consultation Order now You have been selected
Free DVD Order status Your income
Free grant money Orders shipped by priority mail
Free hosting Outstanding values
Free installation Pennies a day
Free investment People just leave money laying around
Free leads Please read