If anyone is experiencing the error "cannot close sent mail" or "cannot open inbox". Here is a pretty good fix from Dave from list.cobalt.com:
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I'm not an expert, but I had a similar problem:

"cannot close mail-sent" or "cannot open Inbox"

I figured out after a while that the user doesn't have
writing rights on its mail folder under users (neomail
creates it the first time...but for some reasons some
time it doesnt work...)

Solved this way for existing Users:

deleted the 'mail' folder under /home/sites/siteX/users/userX/
connected via FTP with the username 'userX' and created
an empty folder called 'mail'.... and now it works fine...

for New users:

I've added a 'mail' dir in the /etc/skel/users/en_US/ directory
...so that the mail folder is automatically created when a
new user is set-up..

I hope it helps... let me know!

Dave

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It worked for me, so hopefully it will for you :)

David Kiley