Don't do this... EVER!

...or else, I'm gonna find you and insult you to death!

Any new technology tends to go through a 25-year adoption cycle.
                                                                   -- Marc Andreessen

Office Open XML (also informally known as OOXML or OpenXML) is an open file format developed by Microsoft and standardized by ECMA (ECMA-376), ISO and IEC (ISO/IEC 29500:2008). Typical Office Open XML file extensions include:

1. .docx
2. .pptx
3. .xlsx
4. .docm.
5. .pptm
6. .xlsm

Different office suites have limited support of Office Open XML format at the time of this writing. Most (if not all) office suites have only partial support for ISO/IEC Office Open XML format at the time of this writing (which means that same document might display differently in different office suites). Even 2007 or newer versions of Microsoft Office have varying levels of support for ISO/IEC 29500:2008 (Microsoft has stated that upcoming MS Office 2013 will support both read and write of ISO/IEC 29500 Strict).

• How can you blatantly assume that every other person on this planet is using 2007 or newer version(s) of Microsoft Office? In case you don't know, there are loads of free and commercial MS Office alternatives.
• I know that Microsoft provides free compatibility packs for older versions of MS Office, but how can you assume that every person has spare time and bandwidth to download and install these packs?
• What about those who use platforms which are not supported by Microsoft Office (e.g. Linux)?
• What about those who can't afford a Microsoft Office license?
• What about those who use cloud based office suites (e.g. Google Docs/Drive)?

Especially when sending documents as attachments to mailing lists, stop being stupid and have sense to use more widely supported formats (such as DOC or PDF) so that everyone can easily access the document.

Mind you, even if you are using 2007 or newer versions of MS Office, you can save a file in 97-2003 format for use in pre-2007 versions of MS Office (and many other office suites), so that you and everyone else can easily access it in most office suites.

Standardization of Office Open XML has been criticized due to many reasons. Quoting Google:
"If ISO were to give OOXML with its 6546 pages the same level of review that other standards have seen, it would take 18 years (6576 days for 6546 pages) to achieve comparable levels of review to the existing ODF standard (871 days for 867 pages) which achieves the same purpose and is thus a good comparison.
Considering that OOXML has only received about 5.5% of the review that comparable standards have undergone, reports about inconsistencies, contradictions and missing information are hardly surprising."

It'll take many (many!) years before Office Open XML will become ubiquitous (when it'll hopefully be supported by all major office suites). Until then, please use more common formats such as PDF, DOC or TXT.