Also, +1 on using Litmus for testing rendering across different clients.
There are also other things to consider when designing email templates and email deliverability in general.
- Always use multi-part, with a nice text part. Not having a text part is a spam flag.
- Keep the ratios of images and links to text low. High ratios are spam flags. Also, most of the time images will not be displayed by default.
- Use common sense, avoiding exclamations and referencing spammy words (eg. buy now!).
- Avoid link shorteners (commonly used in phishing emails)
- Use Litmus to test against spam filters to see where common sense fails.
- mail-tester.com is another nice free tool for checking for spam flags.
- There are also some little things that can trip you up, like having malformed message-Ids where the domain in the message-ID does not match the sending domain.
For more thoughts, we have a best practices (http://documentation.mailgun.com/best_practices.html)
Edit: Also, PLEASE only send emails to people that have given you (like on your website) permission and you have validated the email address with a confirmation link (double opt-in) before sending subsequent emails. Always give recipients the conspicuous and easy ability to unsubscribe.
With the notable exception of iOS Mail.app and OS X Mail, virtually all email clients block images by default.
I think that href images are (correctly) blocked, because they pose a security and privacy risk. That, and the only group who uses them are email marketers. If I want to see your catalog, I'd open your website.
You might have guessed, but yes, I do mark those image-mails as spam (and any mail where I can't find and 'unsubscribe' within 2 seconds).
I wonder if Americans have less trouble with this. When I was last in NY the quantity + obnoxiousness of the advertisements shocked me - we're more used to being blitzed by Germans, not by marketing. Compared to that having an inbox full of brightly colored marketing mails is restful..
Also, it's not very nice to mark messages as spam just because you don't like their design (it affects other recipients). Unless it actually is spam, then mark away.
I send over 150k e-mails per week and have noted that if you bundle together things like "free", ALL CAPS, and exclamation marks, you're going to impact deliverability (as measured through open rates). The key is not just one of these things but actually piling them on top of each other.
Gmail can be very touchy in throwing you to the spam folder if you end up with things like "FREE!!" going on, yet it will be fine with something like "free" or "yay!!"
Gmail's spam filtering seems to be extremely user specific though, so we might see a drop in open rates on Gmail when certain things are included in a mail, but it'll never be absolute and is very hard to objectively prove. You just get a feel for trends over the course of sending hundreds of campaigns.
PS. Also Peter, thank you for starting and sending out the weekly JS, Design and HTML5 newsletters, I'm subscribed and read 'em all, very resourceful.
Just to add: try to make the text part readable and useful too. I've unsubscribed from countless mailing lists because the text was illegible from extremely long URLs thrown in the middle of sentences or useless because the links were missing completely (I use mutt).