I have done these steps 3 times now and I still can't open mail.

Are there disadvantages to Lion as opposed to Snow Leopard?

None of the older PPC apps will run. If you can, you will be better off with Mountain Lion, which, although I don't use it, I've heard is superior.

Google for system requirements and double, at least, the minimum recommended RAM.

I responded on previous page. I actually thought I had it fixed but didnt. Found out later I was getting mail, but couldnt send or reply. ANd the way my mail displayed was messed up too. Could not display TO/FROM?SUBJECT/DATE, etc. "AND" display message-I had to do one or the other. but couldnt have both. The buggy security update changed my mail accounts settings too. Fed up- I called APPLECARE and told them my computer is out of warranty but I was sick of dealing with this thing and they needed to help me fix it since it was their buggy update that screwed everything up! After 3or4 hrs on the phone they finally got it working.No charge!

He had me do a pram reset(press p+r+option+command unitl you hear start up for SECOND time and then release keys. Let start. Then restart after logging in. Apparently this resets hardware.

Then he had me do software reset which is Hold Shift+start until start sound then restart after logging in.

Then I dumped ALL my mails in trash and EMPTIED THE TRASH (did search to find out if any mail files were anywhere else and dumpted them too). Tried to re-install MAIL from original disk and it wouldnt let me. Everything was grayed out and I couldnt check anything. (AppleCare guy even said that was weird) Apparently if it sees mail already in there it wont let you re-install. Even though I didnt have any MAIL files left, for some reason it wouldnt let me use my original disk to re-install. (Guy said he thought something else was wrong with my computer and it turns out it was just their buggy security update afterall!) So I made sure all MAIL files were trashed and trash EMPTIED. I was scared I would lose old emails but he said the computer stores the actual emails separately. Went back to oldest MAIL in Time Machine and restored. Reran combo update & security update in that order. Combo HAS to go 1st. (That's what I did wrong before. I ran security 1st) And it worked. I also repaired permissions at some point but not sure what point that was. If this doesnt work-you might want to check your mail box preferences accounts settings because it changed mine. I didnt even have "mailbox behavior" when it was messed up. It was blank! Now I have it back. Everything works like it did before.