Issue with Mail's Accounts.plist

Hi, I've got the following issue: I used to save e-mails to different directories on the hard drive of an extern file server at work, so as to store them there and so that everyone in the office had access to those correspondenses. Anyway, I work with Apple Mail, and used to use "File>Save as..." in the menu. In the save as dialog window I chose "Formated text" and clicked on "Add attachments" (Note: I work on a German system, so I do not exactly know the phrases used on an English Mac). Worked just fine, everytime I did this with an e-mail that had attachments enclosed to it, I ended up with an rtfd-suffixed file and could always see the attachments by ctrl-clicking the rtfd-file etc. Ever since I have upgraded to Lion however, something goes wrong during the process of saving everything to the hard disk. After trying to find the source almost all day now, I am quite sure the culprit is found in this file:

~/Library/Mail/V2/MailData/Accounts.plist

Twice I replaced this file by renaming it and restarting Mail. In both cases (and after I had set up one mail account again, because that's obviously what the file Accounts.plist is for - storing the information about the accounts used with Mail) the save process worked perfectly well, with the attachments in place and everthing. Then, in the first of my two tries, the trouble came back after I added just one more of the accounts I administer on my Mac. I tried again, and this time I succeded and set up all my accounts as I need them and everything was great and working. After a wee break for celebrating my triumph I played a little more with Mail, changing its settings here and there, just to make it looking nice etc. Then the thought came up, that I should - just to be on the safe side - one last time try and see if saving the mails in the way, I want to do it, still works. It didn't. Same trouble back. What now?