clipped from www.campaignmonitor.com

it seems clear that Postini is placing a lot of importance on the balance between images and text (or HTML) in an email.

Avoid repeatedly sending messages to full or invalid mailboxes. You can do this by tweaking your bounce handling settings for each subscriber list.

Minimize the use of these words and phrases in the subject line, message body, sender address, and reply-to address:

• Use of the word Free (although "free" tends to have more leeway than most other trigger words), $$, XXX, sex or !!! (any excessive punctuation)
• Subject contains "Double Your", "?", "For Only" or "Free Instant".
• TOO MANY CAPS IN THE SUBJECT LINE
• Email contains at least 70 percent blank lines
• The from field appears to not contain a real name, ends in numbers or contains the word friend.
• The email claims not to be spam

Monitor new subscribers in your lists. Set suspicious "spamflag" addresses such as "abuse@" or "spam@" as Inactive subscribers unless you know the subscriber is legitimate.