According to Return Path’s Matt Blumberg these will be the top 5 trends to watch in 2007:

More spam is on its way: The huge increase in the amount of spam being sent has had some serious implications for ISPs and other receivers as they grapple with the increased traffic on their networks. So far, email users have not been overly bothered by this, but that could change depending on how spam filtering technology evolves. The sharp increase in criminal activity by email (phishing, stock scams and so on) could erode consumer confidence if not curtailed. Look for increased activity around this problem by government, industry associations and businesses.

Marketers get to be marketers again: Despite the uptick in
spam, the emphasis on content (i.e., use so-called spam words like
"free") as a way to filter email will wane. This is because it simply
doesn’t work reliably. Instead, receivers will continue to focus on
reputation factors, of which content is only a very small piece.
Complaints will continue to dominate the root factors of delivery
failures, followed by infrastructure issues, unknown users and spam
traps. This means marketers can use the word free again, but they
really have to work hard to please their readers. Marketers who focus
their energy on creating great email programs that recipients love will
reap the biggest gains in deliverability and response.

Rendering will be an even bigger challenge: While filtering
decisions based on content will wane, that doesn’t mean content isn’t
still an issue. In fact, the prevalence of "image spam" is going to
make even more receivers shut down images by default, particularly on
unknown senders. Reputation is still the key, here. Getting known by
the receivers, including participation in all available feedback loop
and whitelist programs, is going to put you ahead of the pack. It will
also become even more important to encourage recipients to add you to a
safe senders list. Finally, designing your email to work with or
without images is going to mitigate problems associated with missing
images.

Email lead generation grows up: The smartest marketers are
going to get more bang for their buck by taking a campaign approach to
email lead generation. Instead of a one-shot blast to new leads,
marketers will create series of emails that engage with recipients on a
deeper level than is possible with one message. These campaigns will
enjoy higher ROI while also preserving the reputation of the advertiser.

Most email house files will not enjoy net new growth: We’ve
had a good thing going for several years now, but as the medium matures
the double-digit growth rates of many lists are likely to wane. Keeping
existing subscribers happy and engaged with your brand is going to be
even more important. This will start with the new subscribers you do
generate as marketers focus on showing value quickly and then
consistently. This also means pulling on all levers to maximize
available growth will be key.

Source: Return Path