This morning, Mark Brownlow asks 3 questions for email deliverability experts:

If you send your email as multipart/alternative and have a text version with wording that differs significantly from the text used in the HTML version, what are the likely deliverability consequences?

None. I don’t think that any of the major filters do comparisons between MIME parts, although they do notice the complete lack of a text part. Even if they did, what they would probably have to be limited to is a size difference between the parts, and that would require some leeway to be given for HTML markup, especially the use of alt tags in images.

I WAS WRONG! There is a hit for this stuff in SpamAssassin. (Note: All of the scores below are default scores for SpamAssassin, but may be altered up or down by any particular user of that filter.)

MPART_ALT_DIFF is defined as “HTML and text parts are different” and results in the following score possibilities:

• Local tests only: 2.498
• Network tests: 1.143
• Local tests using Bayesian filtering: 1.456
• Everything: 0.739

MPART_ALT_DIFF_COUNT is defined the same way, as “HTML and text parts are different” but notices a greater degree of variability. It, predictably, results in (slightly) higher scores:

• Local tests only: 2.899
• Network tests: 1.882
• Local tests using Bayesian filtering: 1.5
• Everything: 1.110

As a rule, I consider anything over 1 to be worthy of correcting and anything over 2 as a major hit in need of immediate correction. I was probably thinking of MIME_HTML_MOSTLY which “Looks to see if the message is mostly HTML content versus normally having plain text parts of nearly equal size” and renders a score of a mere 0.001.

Is there any deliverability penalty if you send just an HTML email but coded as text/html rather than multipart/alternative?

Yes. SpamAssassin, at least, also marks this one pretty high. The test is MIME_HTML_ONLY and is defined as “Message only has text/html MIME parts” with the following default scores:

• Local tests only: 2.299
• Network tests: 1.672
• Local tests using Bayesian filtering: 1.925
• Everything: 1.451

Are there any ISPs where a solid reputation (but no certification) means your images and links are automatically enabled?

This is pretty easy. It is a pretty firm “no.” You should assume that your images aren’t going to be turned on by default. I think AOL still runs its Enhanced Whitelist, but that is a steel cage deathmatch that you have to win to get, and then continually defend. And even better, they don’t tell you when you are added to it or taken off of it. Other than that one exception, no one turns images on by default for anyone without 3rd party certification of some kind.