Newly installed Mail app has just gone WILD!!!

Question

Everything was fine in Mail until I created a new accounts for my new MobileMe email address. Now I have these problems:
1. I can no longer send mail from the .mac aliases, only from the main account. My password, even though identical to the main password, is not accepted on the aliases.
2. I am getting two, three or more copies of every email, and not just new ones; even my emails from last year are being duplicated.
3. When I receive email from one particular address I get a copy every second or so, without limit. This makes my computer very hot and I have to dispose of the extras quickly and then quit as soon as possible. It's a mail merged email with an embedded rota which I have generated myself from a website I help to run and has always performed perfectly well in the past. My internet connection appears to be running at normal speed, however. Today I sent one which appeared as sixteen emails before I clicked the triangle to open the folder. It then became 164! Rebuilding the mailbox has little effect. I think it may have stopped one message duplicating but it certainly didn't work today.
The scariest part of all this (apart from losing email like I'm keeping it in a colander) is that while this email is busy multiplying in my inbox, a selection of my 2008 emails is being sent to people on the list; in the middle of the duplicating file mayhem my husband 'phoned from work to complain that he was receiving my old email at the same sort of rate that I was receiving copies of the rota.
And then my daughter complained the same was happening to her. And then I found I'd had it all emailed to me as well - so now the old emails I haven't lost are sitting there, apparently unread, with incorrect dates and five or six copies of each one! I'm wondering how many of my friends have been treated to a selection of fifty or so of my old emails and what I might have written about them in 'confidence'! Not that I'm given to mouthing off about people in emails but without reading them all I wonder just what might be in them.
4. When I try to delete the second, third and fourth copies of other emails I will suddenly find that other unselected emails, usually from just below in the list, have disappeared along with them and I have to retrieve them from the trash. The alternative is to click 'Undo' but I have to do that about three times before the missing file reappears - and then I have to try and re-delete three files without losing any others in the process. This is hopeless - it's one step forwards and three steps backwards. I also have to memorise which files are around the one I wish to delete before carrying out the action so I will know if anything else has gone.
5. A few days ago I tried to open Mail and found that ALL my email for this year had disappeared. I found some mail from mid July to August in the bin, but all my other 2008 mail was gone. 2007 is still there. I opened it today and found that a random remnant of my 2008 email was in my inbox, but only about a twentieth of the original contents.
6. I'm so glad I still have Entourage installed on my old computer because it's working just fine, apart from the issue I've outlined under point 1. I've retrieved all sorts of important emails from it.
This copy of Mail should be working very well. I only did a fresh system install on a new hard drive about a month ago and everything else is working fine (except a couple of finder windows that just vanished suddenly the other day).
Sorry this is long but I've tried to put several entries into one post. I originally posted this on the wrong forum - it was for MobileMe Mail, but I didn't notice the enclosing folder which was MobileMe on iPhone and iPod Touch; I am using MobileMe on my MacBook Pro. Space Herpes suggested I repost it on this forum to get more help. I had six copies of the email notification of his post so I am going ahead as this HAS to get sorted!

Answer 1

I've left it for a few days before responding because for some reason I hardly got any email for a couple of days and it didn't seem to be completely fixed at first as my rules took a while to kick in again and some junk appeared in my inbox. I've also been too busy to worry about it this week - daughter is starting Uni at the weekend and it's all a mad panic to set her up to move out! Apologies that I posted this on the first thread where this question went before I was prompted to move it to a more appropriate place. I didn't realise I was on the wrong board.
I have had a fair amount of messages today and I'm pleased to say I've only had one copy of everything and the subjects are back to normal - so thank you very, very much as you seem to have solved my problems. I notice that I only have the account for my [email protected] email address and I do wonder whether I need to have an account for the xxxxxx.me.com address as well? Was that possibly responsible for some of the duplicates? Are they interchangeable in Mail?
Despite the fix I do still have the corruption in my inbox, though. I've lost many, many emails and all the problems I mentioned before seem to be permanent in my 2008 mail. I have loads of duplicate copies and the subjects are all mixed up. I also have quite a few recent messages that didn't get downloaded from the server because it thought I was offline.
On my old computer I have everything as it was before. Would it be possible to copy them across somehow without losing my sent mail from this computer - or should I just be content to go to my old computer if I want to refer to anything I've lost?