why emails get blocked by spam filters

How is it possible that my emails are blocked by spam filters?

I got this question from a few friends and a client and this are the things you can do:

Check your newsletter/email on a spam checker tool
Programmers heaven spamchecker
Lyris contentchecker

The problem for the client was the subject line, in order to get a correct and acceptable subject you need to think carefully about it.
A spam filter recognizes capitals and exclamation marks and blocks your mail on them, so try to avoid that.
Make sure your mail/subject doesn’t contain the words on the list you can find here
And also a few others:

• Try not to use attachments
• Avoid bcc (blind copy) to send your mail
• Don’t create an all image email, this big email, that contains all the text in your email, will probably be blocked. Another important issue is that a firewall sometimes removes images from emails so your recipient gets a blank email.
• So try to create a text-based email. Best thing to do is to provide an html AND text based version of the email.

Good luck.