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
Guaranteed email delivery
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How to disable a domain

Sometimes you might want to make it clear that a domain isn’t valid for email.
Perhaps it’s a domain or subdomain that’s just used for infrastructure, perhaps it’s a brand-specific domain you’re only using for a website. Or perhaps you’re a target for phishing and you’ve acquired some lookalike domains, either pre-emptively or after enforcement action against a phisher, and you want to make clear that the domain isn’t legitimate for email.
There are several things to check before disabling email.
1. Are you receiving email at the domain? Is anyone else?
Check the MX records for the domain, using “host -t mx example.com” from a unix commandline, or using an online DNS tool such as xnnd.com.
If they’re pointing at a mailserver you control, check to see where that mail goes. Has anything been sent there recently?
If they’re pointing at a mailserver that isn’t yours, try and find out why.
If there are no MX records, but there is an A record for the domain then mail will be delivered there instead. Check whether that machine receives email for the domain and, if so, what it does with it.
Try sending mail to postmaster@ the domain, for instance postmaster@example.com. If you don’t get a bounce within a few minutes then that mail may be being delivered somewhere.
2. Are you sending email from the domain? Is anyone else?
You’re more likely to know whether you’re sending mail using the domain, but there’s a special case that many people forget. If there’s a server that has as it’s hostname the domain you’re trying to shut down then any system software running no that server – monitoring software, security alerts, output from cron and so on – is probably using that hostname to send mail. If so, fix that before you go any further.
3. Will you need mail sent to that domain for retrieving passwords?
If there are any services that might have been set up using an email address at the domain then you might need a working email address there to retrieve lost passwords. Having to set email back up for the domain in the future to recover a password is time consuming and annoying.
The domain registration for the domain itself is a common case, but if there’s any dns or web hosting being used for the domain, check the contact information being used there.
4. How will people contact you about the domain?
Even if you’re not using the domain for email it’s quite possible that someone may need to contact you about the domain, and odds are good they’ll want to use email. Make sure that the domain registration includes valid contact information that identifies you as the owner and allows people to contact you easily.
If you’re hosting web content using the domain, make sure there’s some way to contact you listed there. If you’re not, consider putting a minimal webpage there explaining the ownership, with a link to your main corporate website.
5. Disabling email
The easiest way to disable email for a domain is to add three DNS records for the domain. In bind format, they look like:

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Abuse Reporting Format

J.D. has a great post digging into ARF, the abuse reporting format used by most feedback loops.
If you’re interested in following along, you might find this annotated example ARF report handy.

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The secret to fixing delivery problems

There is a persistent belief among some senders that the technical part of sending email is the most important part of delivery. They think that by tweaking things around the edges, like changing their rate limiting and refining bounce handling, their email will magically end up in the inbox.
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.
On the other hand, senders can do everything technically perfect, from rate limiting to bounce handling to handling feedback loops through authentication and offer wording and still have delivery problems. Why? Sending unwanted mail trumps technical perfection. If no one wants the email mail then there will be delivery problems.
Now, I’ve certainly dealt with clients who had some minor engagement issues and the bulk of their delivery problems were technical in nature. Fix the technical problems and make some adjustments to the email and mail gets to the inbox. But with senders who are sending unwanted email the only way to fix delivery problems is to figure out what recipients want and then send mail meeting those needs.
Persistent delivery problems cannot be fixed by tweaking technical settings.

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