Hi Jimlee3,
NO! I explizit stated:
Plesk and it's components and your mail - software
NEED the folders as for example ".spamassassin", "@attachments" and as well the files ".qmail" or "maildirsize".
If you are unsure about what to delete, I rather don't recommend this "manual deletion" - solution and point explizit to the first solution:
=> The fastest way is to
DELETE the
eMail - account over Plesk and re-ADD the account afterwards.