What are some good methods for avoiding spam filters?

We often get asked about any simple steps that can be taken to reduce the likelihood of your campaign being flagged as spam by your recipient's ISP, or your recipients themselves. Even when you're sending an email campaign to subscribers who opted-in, your email can still be flagged as spam by overzealous spam filters. Simply using the word "free" in an email message can often land you in the spam heap.
Listed below are a number of simple tips and techniques you should consider when writing and designing your campaigns:

1. Keep sender addresses as short as possible.
2. Avoid continuous sending of messages to full or invalid mailboxes. You can do this by tweaking your bounce handling settings for each subscriber list.
3. 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 reply-to field is empty
   The email claims not to be spam
   The email contains excessive images without much text
4. Monitor new subscribers in your lists. Set suspicious "spamflag" addresses such as "abuse@" or "marketerspam@" as Inactive subscribers.

Most of the popular and more advanced spam filters out there use these and a whole bunch of other criteria to score your campaign. For example, check out the range of filtering rules for SpamAssassin, one of the most popular spam filters on the web.
Ultimately, the most important thing you can do is adhere to best practices for email marketing. Gain permission, compose relevant content, and deliver messages according to the customer's needs, wants, and preferences.

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