It’s easy to slip from being targeted marketing email to being junk email and getting instantly dumped by the recipient. The spam email filters and junk email traps do not declare their rules (otherwise they wouldn't work). But you should avoid terms like free and cheapest and language that simply sounds inappropriate. Messages that are exceptionally brief can also get junked. 

To avoid your targeted marketing email being treated as junk email by the receiver or automatically by the junk email traps and spam email filters here are some rules to help maximise the chances of being kept off the email spam and email junk mail lists:

1. Focus – don’t send to anybody just because you have their email address.

2. Frequency – don’t get carried away and send emails every couple of days. Once a fortnight is about as often as you should risk (unless they’ve signed up for a newsletter of course).

3. Ensure you have an unsubscribe option.

4. Make sure you declare who the email is from at the bottom of the email.

5. Make sure the email server that you are sending from has “reverse DNS” (ask someone technical) if it doesn't some receiving servers will reject the email.

6. Don’t use robot or auto generated email addresses – they may contain email traps and if they do you might get tarred with a spam email brush.

7. Send a test email message to a few folks to make sure it does not trigger their spam traps (ie don’t offer FREE VIAGRA). Some online marketing email agencies will offer this as a service.