Most receivers use reputation-based spam filtering, either purchased, home grown, or a combination. Spam filters have different ways of accomplishing the goal of protecting their users from unwanted email, but they all have some basics in common.
ISPs' response to undesired email
For email that is sent from an IP address or domain with poor reputation and poor or missing authentication, ISPs typically respond in increasing levels of rejection:
- Throttling or temp-failing
- Hard bounce
- Hard block
Depending on the ISP, the blocks disappear over an arbitrary time (24 - 72 hours, generally) if the problem that caused the block stops.
- If a block is issued and does not go away, you must open a ticket and the resolve the problem to the satisfaction of the ISP before the block is removed.
- Of the four major ISPs (Yahoo, Gmail, Hotmail, and AOL), only Gmail has no way to open tickets or ask for help.
It is always easier to avoid being blocked by doing the required work for ramp up and maintenance than it is to get a hard block removed.
Spam filters, block lists, and blacklists
The terms "spam filter" and "block/blacklist" can be used interchangeably. Generally, Gmail, Hotmail, Yahoo, and AOL use mostly home-grown spam-filtering technology. Because they are mailbox providers, they have access to a huge amount of data about their own users' email behavior. They prefer to use their own data because it is more accurate for their situations.
Providers like Comcast, Roadrunner, and Sky use commercial spam filters like Cloudmark and Spamhaus.
Modern spam filters are sophisticated and fast. If your email is blocked by any spam filter, it can be time-consuming and expensive to resolve. The impact depends on many factors, including how widespread the use of the filter is. There are hundreds of block lists in the world, but only a few have a broad impact. For example, a listing on the Spamhaus block list has enormous reach, whereas a listing by SPEWS might be ignored.
Spam filtering technologies
Words or phrases that can trigger spam filters
When you create an email, there are certain phrases or characters that can be flagged by spam filters. You should avoid these words and phrases.
Email subject linesSome of the more commonly used phrases that ISPs and mail clients filter out include:
|Free!||50% off||Click Here||Call now!|
|Subscribe||Earn $||Discount!||Eliminate Debt|
|Double your income||You're a winner!||Reverses Aging||"Hidden"|
|Information you requested||"Stop" or "Stops"||Lose Weight||Multi level Marketing|
|Collect||Amazing||Cash Bonus||Promise You|
|Credit||Loans||Satisfaction Guaranteed||Serious Cash|
|Search Engine Listings|
|Act Now!||All New||All Natural||Avoid Bankruptcy|
|As Seen On…||Buy Direct||Casino||Cash|
|Consolidate Your Debt||Special Promotion||Easy Terms||Get Paid|
|Guarantee, Guaranteed||Great offer||
Give it away, Giving it away
|Meet Singles||MLM||No cost, No fees||Offer|
|One time||Online pharmacy||Online marketing||Order Now|
|Please Read||Don't Delete||Save up to||Time limited|
|Unsecured debt or credit||Vacation||Viagra||Visit our web site|
|While Supplies last||Why pay more?||Winner||Work at home|
|You've been selected|
Using quotation marks, dollar signs and exclamation points in subject lines frequently trigger mail filters, as well as using all capital letters (shouting). Avoid toll-free numbers in the subject line, because that also causes your email to be filtered out by many spam filters. And never use a font size larger than 2+, or you could trigger some Spam filters.
Junk Mail Content filter
The Junk Content filter works by looking for key words. This file is a description of exactly which words the filter looks for and where the filter looks for them.
|From is blank||Subject contains "advertisement"||Body contains "money back"||Body contains "cards accepted"|
|Body contains "removal instructions"||Body contains "extra income"||Subject contains "!" AND Subject contains "$"||Subject contains "!" AND Subject contains "free"|
|Body contains ",000" AND Body contains "!!" AND Body contains "$"||Body contains "for free?"||Body contains "for free!"||Body contains "Guarantee" AND (Body contains "satisfaction" OR Body contains "absolute")|
|Body contains "more info " AND Body contains "visit " AND Body contains "$"||Body contains "SPECIAL PROMOTION"||Body contains "onetime mail"||Subject contains "$$"|
|Body contains "$$$"||Body contains "order today"||Body contains "order now!"||Body contains "money-back guarantee"|
|Body contains "100% satisfied"||To contains "friend@"||To contains "public@"||To contains "success@"|
|From contains "sales@"||From contains "success."||From contains "success@"||From contains "mail@"|
|From contains "@public"||From contains "@savvy"||From contains "profits@"||From contains "hello@"|
|Body contains " mlm"||Body contains "@mlm"||Body contains "///////////////"||Body contains "check or money order"|
Adult Content filter
The Adult Content filter works by looking for key words. This file is a description of exactly which words the filter looks for and where the filter looks for them.
|Subject contains " xxx"||Subject contains "over 18"||Subject contains "over 21"||Subject contains "adults"|
|Subject contains "adults only"||Subject contains "be 18"||Subject contains "18+"||Body contains "over 18"|
|Body contains "over 21"||Body contains "must be 18"||Body contains "adults only"||Body contains "adult web"|
|Body contains "must be 21"||Body contains "adult en"||Body contains "18+"||Subject contains "erotic"|
|Subject contains "adult en"||Subject contains "sex"||Body contains "xxx"||Body contains " xxx!"|
|Subject contains "free" AND Subject contains "adult"||Subject contains "free" AND Subject contains "sex"|
For more information, see the following websites:
- Spamhaus Frequently Asked Questions
- Symantec Cloud Based Protection
- Microsoft™ Office 365 email anti-spam protection
- Apache SpamAssassin Frequently Asked Questions
- Totera Cloud Solutions (formerly MXLogic at
- DNSBL (DNS-based blacklist) of NiX Spam (used primarily by German ISPs)
- Fundamentals of Cloudmark Technology
- Gmail Bulk Sender Guidelines
- Cisco Senderbase
- Cisco Senderbase Spam Overview
- Proofpoint Email Protection Product Suite