Secrets, a clever little System Preference Pane that makes tweaking “hidden” features of Mac OS X easy (see earlier Hawk Wings post for more), has been updated to 1.0.6 and is now compatible with Snow Leopard.
Secrets provides easy assess to many of Mac Os X’s settings that you can otherwise only change by messing around in Terminal with long command strings, which is not everyone’s cup of tea.
It includes tweaks for many Mac core and a wide range of third-party apps. The most popular tweaks across all the apps are listed separately as well.
Of course, here we are most interested in its options for Mail.app:
Secrets Mail Preferences
As you can see from the screenshot, Secrets allows you to
- specify a default BCC email address
- force Mail to display messages in plain text
- set the Bundle compatibility and enable bundles
- enable and disable the data detectors
- switch the new (annoying) Snow Leopard behaviour of including names in copied email addresses on and off
- set a sent mail sound
- specify a minimum for HTML messages and a preferred text encoding
- request read receipts
- set the interval for refreshing Mail’s RSS feeds
- Decide whether to display attachments inline or not.
And more.
Some people will think of it as a hack and might be wary. However it comes with the reassurance that Alcor, the developer also (once) behind Quicksilver, is its creator. That’s a strong pedigree.
Secrets is freeware and available from the Blacktree web site 
.
UPDATE: I read on TUAW that the Blacktree site is overloaded. Secrets is also available from the app’s page on code.google.com.
We see a lot of confusion about Secrets on its Google Group and in various forums online.
Secrets does NOT change anything except plist files on your computer. It doesn’t modify programs directly, it merely enables settings that programmers have already included but don’t provide GUI interfaces to.
All of the settings mentioned above for Mail.app are things Apple has put into place, they just don’t provide a mechanism to control. That’s where Secrets comes in.
You can think of it as a GUI interface for all those “defaults write com.company.app” things you see on macosxhints.com or elsewhere.
That’s a great explanation of why no one should be afraid to use it. Thanks, Nick.
I will follow that up by saying that there may be very valid reasons the programmers don’t offer up an interface to turn on these features: they could break things and typically they’re completely unsupported.
That being said, anything that stands the potential to cause harm will almost always be classified as a “dangerous” secret, and will show up appropriately in Secrets (usually highlighted in red). But most of the stuff is just harmless tweaking.
Its almost worth noting that Secrets is community driven; if you know of a secret that you can’t find in Secrets, you can go to secrets.blacktree.com and submit it as a new one.