If your Mail.app is crashing unexpectedly or you're planning on buying a second Mac (or have another Mac already, but it doesn't catch Junk mail as well as your other Mac does), you can work with a file Mail.app uses to track your Junkmail settings. It's also a good idea just to have a copy of these settings around for a rainy day.

Archive:
First, let's back up Mail.app's junkmail settings. Backing up your Mail.app Junkmail settings is easy. .

We'll begin by finding your Junkmail settings. Mail.app collects these settings in a file with the title "LSMMap2. This file is located in the Mail folder in your Library:

~/Home > Library > Mail > LSMMap2

You'll want to make a copy of that file because you can't take this file from the Mail folder. Do this by clicking it once and selecting File > Duplicate (+D). This will create an exact copy of LSMMap2 and label it LSMMap2 Copy.

Next, move LSMMap2 Copy somewhere on your hardrive where you keep other backups of important files (such as Addressbook, etc) for safe keeping.

Troubleshoot:
If your Mail.app is crashing unexpectedly, or crashes when you try to read a particular message, sometimes trashing the LSMMap2 file can help. That's why we have the backup saved on your HD.

Replacing the LSMMap2 file with your backup file is easy. First, quit Mail.app. Then locate your LSMMap2 file within ~/Home > Library > Mail folder. Next drag it to the trash. Finally, find your backup copy of LSMMap2 (which should be named "LSMMap2 Copy"). Next make a copy of this file, and rename it "LSMMap2" which will match the one you just tossed out. Finally, drag this file back into ~/Home > Library > Mail.

That's all there is to it, and it just might fix the crashing issue you're experiencing with Mail.app.

New Mac?
If you have another Mac, new or old, that doesn't handle Junkmail as well as on of your other Macs, you can use the LSMMap2 file to instantly beef up its handling of Junkmail. After all, you might have spent a year marking spam and you don't want to go through that with your new Mac.

The process is basically the same as described above in the Troubleshooting section. Just grab a copy of your backed up LSMMap2 file and move it to your new Mac. Short and sweet, but it will save you a lot of time.

Maintenance
Some Mail.app gurus suggest making fresh copies of the LSMMap2 file. The theory is that Mail.app will run more smoothly if you do this regularly. I do it, but I can't say for sure it it helps. But since it only takes a second, I recommend it.

What you'll want to do here is make a copy of your LSMMap2 file and then toss out the original. You'll be left with a file called "LSMMap2 Copy" which you'll want to rename "LSMMap2" (you will want to make a new copy of this file and replace your backup LSMMap2 for future use if necessary).

I hope some of these tips help you with Mail.app.

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Don't forget How to Buy a Mac.