If you are talking about delivery, I believe the most important components of delivery are:

1. Having relationships with ISP’s which a lot of the email providers do not. That is why we partnered with Constant Contact; they have one of the highest (if not the highest) delivery rates in the industry due to the relationships they have established with ISP’s combined with their standards for their users. This works to resolve spam issues for both HTML and text based emails.

2. Knowing the “spam value” of your email before you send it. The more responsible email service providers integrate and spam check mechanism in the software that allows you to see how a spam filter might view your email before it is sent, allowing you to make corrections before you send and not after. Again, Constant Contact does a great job with this.

With regard to effectiveness in terms of call to action, I would say it depends on your market. If you are mailing to pregnant women, or artists, you would fare much better to use HTML emails. If you are sending updates to techies or software developers about vulnerabilities or patches, text email may be your best bet. Also, with the advent of blackberries, PDA’s, and the like, it is optimal to send a text version of your HTML email allowing for maximum deliverability. Not to sound like a broken record, but that is another feature that I love about Constant Contact. The interface allows you to enter both a text and HTML version of what you are sending.

Great questions – I hope that is helpful.

Anthony