How to export an email from Mail application

I remember solving an issue similar to this, one or two years ago. I don't have a Snow Leopard system handy so unfortunately I cannot be absolutely sure as to the process required. The steps below are what I would do for a Lion system with some notes attached where possible adaptations would be required.

As you suspected, the problem is due to a file becoming corrupt.

Solution:

1. Quit Mail if it is open.
2. In Finder, go to ~/Library/Mail and backup the folder contents
3. While still in ~/Library/Mail, open the subfolder called V2. Inside that folder there should be a folder called MailData, open that folder. (If you do not see a folder called V2 or a folder called MailData, it is because the folder structure may have changed between OS versions. Proceed to step four in this case.)
4. There are a few possible files which may need to be removed from the User's mail folder. In the MailData folder (or if you do not have a MailData folder) search for and move the following files to the Desktop:

(I suggest removing them one at a time and relaunching Mail after removing each file. If it works, your job is done.)

• DefaultCounts
• LSMMap2
• Envelope Index -- (If you have to delete this file, delete the following two at the same time)
• Envelope Index-shm
• Envelope Index-wal

A possible pitfall to this method: There is a possibility all messages in folders could get marked as Unread. Depending on the users workflow, this could also be a problem; though a solvable problem through sorting read and unread messages temporarily into different folders and recombining later.

Try this.

   using terms from application "Mail"
    on perform mail action with messages theMessages for rule theRule
        -- The folder to save the attachments in (must already exist)


        -- Save in a sub-folder based on the name of the rule in Mail

        set subFolder to name of theRule
        tell application "Finder"
            set attachmentsFolder to ((path to home folder as text) & "Dropbox:Attachments") as text
            if not (exists folder subFolder of folder attachmentsFolder) then
                make new folder at attachmentsFolder with properties {name:subFolder}
            end if
        end tell
        tell application "Mail"

            repeat with eachMessage in theMessages

                set {year:y, month:m, day:d, hours:h, minutes:min} to eachMessage's date sent
                set timeStamp to ("" & y & "-" & my pad(m as integer) & "-" & my pad(d) & "-" & my pad(h) & "-" & my pad(min))

                try
                    -- Save the attachment
                    repeat with theAttachment in eachMessage's mail attachments

                        set originalName to name of theAttachment
                        set savePath to attachmentsFolder & ":" & subFolder & ":" & timeStamp & " " & originalName
                        try
                            save theAttachment in file (savePath)
                        end try
                    end repeat

                    display dialog subFolder
                end try
            end repeat

        end tell
    end perform mail action with messages
end using terms from

-- Adds leading zeros to date components
on pad(n)
    return text -2 thru -1 of ("00" & n)
end pad

I have posted a more general Automatically Save Attachments in Mail.app on my blog