Multipart MIME cheat sheet

I’ve had a couple of people ask me about MIME structure recently, especially how you create multipart messages, when you should use them and which variant of multipart you use for different things. (And I’m working on a MIME parser / generator for Abacus at the moment, so it’s all fresh in my mind)
So I’ve put together a quick cheat sheet, showing the structure of four common types of email, and how their MIME structure looks.

Simple plain text

text/plain

Plain text with a PDF attachment

multipart/mixed

text/plain
application/pdf

HTML with a plain text fallback

multipart/alternative

text/plain
text/html

HTML with embedded images and plain text fallback

multipart/alternative

text/plain
multipart/related

text/html
image/gif
image/gif
image/jpeg
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Poor delivery can't be fixed with technical perfection

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In my own work with clients, I usually start with making sure all the technical issues are correct. As almost all spam filtering is score based, and the minor scores given to things like broken authentication and header issues and formatting issues can make the difference between an email that lands in the inbox and one that doesn’t get delivered.
I don’t think I’m alone in this approach, as many of my clients come to me for help with their technical settings. In some cases, though, fixing the technical problems doesn’t fix the delivery issues. No matter how much my clients tweak their settings and attempt to avoid spamfilters by avoiding FREE!! in the subject line, or changing the background, they still can’t get mail in the inbox.
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Is there really any difference in permission between purchasing a list of “qualified leads” and automatically adding anyone who makes a purchase at a website to marketing lists? From the recipient’s perspective they’re still getting mail they don’t want, and all the technical perfection in the world can’t overcome the negative reputation associated with spamming.
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This is a gross misunderstanding of the reasons for bulk foldering and blocking by the ISPs. Yes, technical behaviour does count and senders will find it harder to deliver mail if they are doing something grossly wrong. In my experience, though, most technical issues are not sufficient to cause major delivery problems.
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