Home Forums Networks & Cross-Platform Networking Code? Gibberish?

I am going t attempt to sum up what has been determined so far.

1. Thunderbird is successfully decoding MIME that is tripping up Mail. Since Mail and Thunderbird each use their own different MIME decoder and it is possible Apple's MIME decoder is confused by a stray non-ASCII character that Thunderbird's MIME decoder simply ignores.
2. This problem has appeared in Mail in the past but Apple fixed it.
3. Since others are not reporting this problem it would appear to be peculiar to Mavericks Mail on your computer.
4. The failure of Mail's MIME interpreter may be the result of some out of date detritus from a previous version of OS X or a problematic kernel extension, plug-in, or some other apparently unrelated code snippet in your system.
5. I contend something in your email provider's server is contributing to the situation which makes all this even more difficult to troubleshoot.

I don't know how far you want to pursue this, but here are some thoughts I have had in no particular order.

• There is a handy dandy free troubleshooting app Etrecheck that is useful at identifying potentially problematic extensions, plug-ins, etc. You might try running it to see if anything pops up. I know it helped me identify a couple of out of date kernel extensions that were causing me some issues.
• I was recently having some inexplicable problems on one of my Macs that at the suggestion of the Genius Desk at the local Apple Store they were cured by flushing all — and I do mean ALL — of the various cache files in the entire system. That involved going to /System/Library/Caches, /Library/Caches, and ~/Library/Caches and deleting all the files and folders in each of those locations. It might slow things down a bit until all the caches are rebuilt, but who knows it might, just might solve your issue. (It is almost scary how many grow back so rapidly when you do that.)
• Escalate the issue by gathering up a couple of sample emails (raw text versions so non=printing ASCII would show up in a Hex editor), a System Report and a careful description of that problem and what you have done to troubleshoot it and contact AppleCare and if necessary be prepared to request the issue to be escalated. The more data you have available the better. Personally I would put all the files in a folder on iCloud so you can share the data with them rather than attempting to email all of that because the files will get LARGE.
• In the interim, if you are not too thrilled with Thunderbird you might try Airmail 2. It is available through the App Store. They no longer offer a free trial of the released version, but you can get a free download of the 2.1 beta which I am using and it is very stable. I have come to prefer it to Mail — much to my surprise.
• If all else fails there is always the nuclear option.
  1. Back up everything, cloning is recommended. Gather up any current application install DVDs installation keys etc. Make a list of all your download installs (if they are from the App Store it will remember for you.) Be sure your Keychain, Calendars, Contacts, Photos, Tunes, etc. are all in iCloud.
  2. Boot from the Recovery Drive (either option boot of boot holding down ⌘R) and erase the Hard Drive then Install Mavericks.
  3. Go through the setup routine, but do NOT restore from either the Time Machine or clone backups.
  4. Reconnect to iCloud to restore all your passwords and other settings (you might hold off on Mail and set that up completely from scratch — just in case).
  5. Copy any data files not in iCloud from the clone by drag and drop or from the latest Time Machine backup.
  6. Re-install your non-App Store applications — ONE AT A TIME and carefully testing Mail after each.
  7. Re-install any kernel extensions, plug-ins, anti-malware applications — ONE AT A TIME and carefully testing Mail after each.
  8. Don't re-install any application, kernel extension, plug-in, etc. UNLESS YOU REALLY NEED IT.
  9. I Hope it does not come to this as it is a pain in the nether regions of the anatomy and very time consuming. Even worse it does not guarantee a fix.