Mail.app seems to get slower and slower. Plus it crashes at least once a week. As I started looking around the web to find a solution I ran across this post from Tim Gaden at his blog: http://www.hawkwings.net/2007/03/01/a-faster-way-to-speed-up-mailapp/

He describes this solution:

Open Terminal, then run these three commands:

ls -lah ~/Library/Mail/Envelope\ Index
sqlite3 ~/Library/Mail/Envelope\ Index vacuum
ls -lah ~/Library/Mail/Envelope\ Index

I’ve just ran it myself and it only dropped the size of my index from 73MB to 60MB, but it DID speed up Mail considerably. I’ll have to wait and see if it helps the crashing.