It's been 2 years but I just wanted to mention for anyone looking at this post that background-size is no longer stripped in Gmail. At least not in Gmail on the web which is what I assume Mark was referencing 2 years ago.

Looks like /deep/ and ::shadow are no longer supported in Chrome as of version 63. As best as I can tell there is no way to remove the broken image icon in Chrome now.

https://developers.google.com/web/updates/2017/10/remove-shadow-piercing

The good news is that you can reach 95% of your audience with video which plays at the moment of opening

Where are you getting this number from? Considering video doesn't work in the Outlook desktop client, your 95% figure doesn't sound accurate.

Thanks for the reply Justine!

Are there any resources you'd recommend on verbiage to explicitly avoid? The article you linked does a great job of talking about spam verbiage as it relates to the user who is actually reading it. But I'm mostly worried about the corporate environment. Where a spam filter is going to judge the content of my email before my subscriber ever sees it, with little or no regard to what my subscriber has to say about it.