Why has my Mail app disappeared from my MacBook Pro?

Question

If i try to open the Mail app from my applications folder I get the following error message, "You can't use this version of Mail with this version of OS X"
As far as i remember the mail app disappeared since I accepted a security update.
Grateful for any help in retrieving my mail and mailboxes......

Answer 1

!! WARNING !!
about the September 2012
"Security Update 2012-004"
If you've got "Mail.app" in a folder OTHER THAN the root "Applications" folder,
MOVE "MAIL.APP" TO THE APPLICATIONS FOLDER
**BEFORE** INSTALLING THE "004" UPDATE!!
Because *IF* you use OSX's integrated Software Update app from the Apple menu to do the "004" updating, and *IF* your Mail.app is NOT in the Applications folder, then buddy, you've got some sitdown fixit time ahead of ya and/or some internet tie-up time downloading the "004" update file itself.
Been through it all, already.  In fact, just got done getting my Mail back up & running. And I tried everything short of attempting to unpacking my "Mail.pkg" receipt file using... whatever... even Pacifist couldn't get it out. So that fixit "Plan D" died quick. And I'm not savvy enough to use Terminal commands to do "Plan E," so that SUDO stuff was my brick wall.
So I compressed my entire "Mail" subdirectory that contain all my emails, for safekeeping during the required surgery, or whatever would work, leaving the original Mail directory where it was. In case something bad happened in all my fixit whatevers, the one thing I *didn't* want to cross paths with was a hosed Library -> Mail folder. Then I'd be pssd. But I can wipe out that ZIP now 'cause all's well now. After 6 hrs of downloading thru a mobile broadband hotspot running at only 14KB/sec speed due to my having gone over my T-Mobile hotspot plan's bandwidth amount (throttled-down after reaching the limit, tho not cut off completely. Kinda nice, but it's a huge speed drop).
I tried moving the two Mail.app versions around, compressing them & then trashing the original so they wouldn't be recognized, blabla, even booted my Mac in Safe Mode & reinstalled the "MacOSX 10.6.8 (Snow Leopard) Update Combo.dmg" update file, which in the past has fixed many problems. But after restarting, not this one.
Like a dummy who ignored knowing better, I had my Mail.app in a folder other than the Applications folder, with only an alias to that being in the Applications folder. NOT GOOD ENOUGH. The updater don't see that as nuthin special or useful.
So if you've got, like I've got, "old" icons for Mail.app (v4.5) in your Finder toolbar & in your Dock that all point to "Mail.app v4.5" in whatever folder you've got that in, other than in the Applications folder (where Apple apparently REQUIRES that it be located, and ONLY there, at least for OSX updating purposes), then after you download & autorun that "004" updater using Software Update, NONE of your pre-existing Mail icons will work. And MOST aggravatingly, **NOR** will your brand-new "Mail.app v4.6" work that came out of the "004" updating installation.  So basically, your Mail.app's... all of 'em.... are hosed. But only if you run the "004" update without your "Mail.app v4.5" being located in your Applications folder and ONLY in your Applications folder. And duplicates of Mail.app, scattered around your HDD??  Always a problem with many Apple apps.... The Apple software engineers apparently like 'em to be in the Applications folder, and in ONLY there, and ONLY the latest version, and ONLY 1 single copy of each on your entire HDD. They just don't seem to stress that enough, it appears. Kind of an oversight for them to not emphasize that, IMO...
My recommendation for this particular September 2012 "004" update:
If you run Software Update, and if you see the "Security Update 2012-004" listed as available for download, UNCHECKMARK IT.  DO NOT DOWNLOAD IT USING SOFTWARE UPDATE !  Instead, download the actual update file itself.
Same thing, only a different way of doing it. Most importantly, you'll be skipping the auto-installation that Software Update performs.
AND you'll have that update file on your HDD for future fixits, should a nasty "unfixable" problem come up.
First, #1. Do a filename search for "Mail" (Option-Command-Spacebar). Scroll down to where the "Apps" are located in the list under the "Kind" column heading. First, find your Mail.app that's v4.5. Get that bugger into your Applications folder if it's not already there. Next, Trash all other "Mail.app" apps you see in that same list that you may have elsewhere on your HDD.
#2. Go here: https://support.apple.com/kb/DL1586
And download the actual "004" DMG update file: "SecUpd2012-004.dmg"
And of course, it's a biggie.... 270MB... so watching grass grow may be involved here if you've got a slow connection (go to a MacDonalds & do it, like I shoulda done).
3. After downloading, I'd recommend closing down all apps, emptying the Trash, and restarting.
Then, after restarting, run Disk Utility's "Repair Disk Permissions."
Then, doublecheck that the Mail.app version 4.5 is in your Applications folder.
THEN fire up & install "SecUpd2012-004.dmg."
Reboot.
After the reboot, your Mail.app version 4.5 in the Applications folder will have become Mail.app version 4.6 in the Applications folder, and all will be well again.
And you can dump that zipped-up "Username -> Library -> Mail" folder then, too.
Happy Mac'in!
Kevin Kendall
Macbook 7,1
(Apple's very last all-white Macbook model)
2.4GHz - 256GB Crucial SSD - 8GB Crucial RAM
OS 10.6.8 Build 10K549 + Win 7 Ultimate thru VMWare Fusion v5.0.1