People writing mathematics in emails, like researchers in mathematics or
physics, have probably encountered this difficulty to properly format complex
mathematical formulas. The most common technique is just to write text with
LaTeX-like or ASCIIMathML-like syntax and hope that the recipient will just
understand the expressions. Obviously, this is not really convenient to write
and read, some errors may happen and result in misunderstandings between the
sender and the recipient. There are other classical issues like how to write
the math (special syntax? math panel? handwriting recognition?), accessibility,
rendering quality etc Of course, these issues are well-known and expected to be
addressed by MathML. Since HTML is a common format for email and MathML is now
part of HTML5, this is clearly a good candidate to solve the problem of
mathematics in emails.

The idea to use MathML in emails is not new and was already suggested in a
screenshot from the Mozilla MathML Project more than 10 years ago.
Thunderbird has been able to render MathML in
newsfeeds for a long time, provided that the author served his content as
XHTML. I may also mention Amaya, which added support for sending a document by
email in 2007, although I have never figured out how to configure it to send
emails. Two years ago, I tried without success to fix a bug to display XHTML
attachment inline and which could be a partial solution to the problem.
Finally, one year ago Bob Mathews (from Design Science) asked me about the
status of MathML in Thunderbird, and I could unfortunately not give him a
better answer than what is in the present paragraph. But I hoped that MathML in
HTML5 will change the situation.

Indeed, while I was working on some MathML-in-clipboard patches, I realized
that it is now possible to paste MathML inside an email. After further
discussions with Bob Mathews, Paul Topping & David Carlisle, I've been able
to do more testing. The situation is the following:

- Thunderbird can send emails containing MathML and render them
correctly.
- Apple Mail (used in Mac OS X and iOS) can receive emails containing
MathML and should render them correctly since MathML
is enabled in Apple's products.
- Microsoft Outlook does not render MathML in emails. However the rendering
is based on Microsoft Word which has MathML support. Basically, Thunderbird
sends MathML in HTML5 and Word displays MathML after an XSLT conversion
into Microsoft's own OMML format. Hence Microsoft might be able to do
something not too complicated to make the whole stuff work.
- Web Mail Clients like Gmail or Zimbra seem to filter the MathML in emails
and so do not render it correctly. If this filter is removed, they can
certainly let the browser do the rendering job or use MathJax to do so.

Now let's consider a basic example about how to send MathML in emails, using
Thunderbird. One of the issue is that Gecko's editor has really been designed
with only HTML-editing features in mind and if you start editing MathML formulas you are
going to get some invalid markup messages or other troubles. And of course
Thunderbird does not have any math panel or other WYSIWYG tools to write
mathematics. However it might not be too difficult to write an add-on to add
MathML editing features in Thunderbird like BlueGriffon's
add-on or Firemath (these add-on
might even be installed without too much trouble in Thunderbird). Or one can of
course use one of the existing
tools to generate MathML and just paste the code in Thunderbird. Here I'm
going to use the `itex2MML`

filter. So first write your mail in a
separate text file:

mail.txt
Hi Matthew, I just read your email about the behavior of the factorial function and harmonic series for large values of $n$. If you denote by $\gamma \approx 0.5772156649$ the Euler's number, by $e \approx 2.7182818284$ the Euler's constant then you have the well-known Stirling's approximation:
$$n! = \sqrt{2 \pi n} {\left( \frac{n}{e} \right)}^n \left( 1 + O \left( \frac{1}{n} \right) \right)$$
where of course I use the classical constant $\pi \approx 3.1415926535$. We also have the following asymptotic expansion:
$$\sum_{k=1}^n \frac{1}{k} = \ln(n) + \gamma + O \left( \frac{1}{n} \right)$$
I hope that this answers your question.

then call `itex2MML`

to replace the LaTeX code by
`<math>`

elements:

cat mail.txt | itex2MML > mail.html

Write a new mail in Thunderbird and use the menu "Insert ; HTML" . David
Carlisle told me that you have to be sure that the "send as HTML" is enabled if
it does not show up. Then just copy the `mail.html`

source into the
window:

Once you click the insert button, the MathML should be automatically
rendered in Thunderbird:

When your email is ready, just send it as usual! Here is how it appears on
an iPod:

Let's just hope that other mail clients will support MathML in emails!