/home/garberw/.thunderbird/****.default/Mail/Local Folders/System is a symbolic link to /var/mail/garberw. When I use thunderbird my email to root appears under Local Folders in a subdirectory System with separate emails. The problem is that /var/mail/garberw is a single file which is reinitialized so that every time a new email arrives they are all marked as unread.
There is probably nothing wrong with the permissions on /var/mail/garberw.
THE ORIGINAL METHOD (completely different):
I can not use postfix to forward the email to my external email provider, att.net, since they (as far as I can tell) bounce the “relay”; i.e. they do not allow email sent from root to have the sender name replaced as my email address so it looks like I sent the email to myself because swapping the sender is a common way of sending spam. I had this solution working then suddenly it stopped working indicating that they changed their policy and made it illegal.
Cancel any email account wizards, then access the account settings (AltE to open the “Edit” menu, then “Account Settings”). From there, click on the “Account Actions” drop-down and select “Add Other Account…”. Check that “Unix Mailspool” is selected and click on “Next”. Fill in an identity — the local part of the email address will be used by default as the spool name (the file in /var/spool/mail which contains the email you want to read) — and click on “Next”. Skip the “Outgoing Server Information” page by clicking on “Next”. Check the account name and click on “Next”. Finally, click on “Finish” and review the account settings. You should then be able to retrieve the email from the local mailbox.