As the administrator for several MTAs I've received my fair share of support requests for "unreadable" e-mails. They generally have an attachment called winmail.dat. This is what all (for exceptions see the rest of this blog post) non-Outlook MUAs can display, when they receive an e-mail with the application/ms-tnef (or application/vnd.ms-tnef) MIME type. TNEF is a propietary format cooked up in Redmond. When chosen, the e-mail in question isn't sent as text/plain or at least text/html, but the entire e-mail is put into an TNEF-encoded attachment, most often called winmail.dat, in which you (most commonly) find the content of the e-mail in RTF, possible attachments are also put into the winmail.dat.
The easieast solution for all involved parties is to convince the sender, that her Outlook and/or Exchange setup needs fixing (disabling TNEF in Outlook and Exchange 2003). Sadly this won't work all the time. The other solutions are to either filter the e-mails on your e-mail server and run them through something like ytnef_smtpd.py (please note, that this might be legally problematic for you as a service provider, opening an e-mail, even automatically, is a violation of communication privacy rights in many jurisdictions). Or, if the legal problem is too big or can't be solved through an local server installation (e.g. because there is no server running 24/7), you're left with either using Outlook (probably what Microsoft intended) or you can use a plug-in/add-on for your client. I've tested the LookOut add-on for icedove and it seems to work well. There are of course plug-ins for many other clients, but I can't honestly tell whether they work or not.
[UPDATE 2011-09-20]: Fellow Debian Developer Josselin
Mouette wrote me, that Evolution is capable of reading
TNEF-encoded e-mails out of the box, but he also said that [TNEF e-mails
should] be banned by all means
.[/UPDATE 2011-09-20]
Now, before I close, please always try to convince the sender/postmaster to fix their setup. It has only advantages: text/plain e-mails are smaller, they're less likely to contain malicious code (i.e. no parts hidden away from the virus scanner on the mail server (amavis can read TNEF e-mails, if libconvert-tnef-perl is installed)) and all recipients can open the e-mails with the client of their choice.
