Manually Rebuild Apple Mail. Fix seeing HTML Code, connection errors, etc in High Sierra.

Apple Mail has been a finicky bit of software since its inception. Some love it, some hate it. One problem we've seen recently in V10 with High Sierra is that emails display [the usually] hidden HTML code. This can be frustrating. Some have 'solved' it using Apple Mail's built-in "rebuild" option, but the problem seems to return pretty quick.  Here's how you can solve this issue and other weird mail issues too.

1. Open System Preferences > Internet Accounts> iCloud > UNCHECK "Mail"
2. Inside Apple Mail > System preferences>  disabled Junk mail filtering. 
3. Inside Apple Mail > Export your local mail to keep a copy safe: Highlight the mailbox in the list on the left and click "Mailbox," then in the upper menu click "Export Mailbox," then choose a place to save the folder. Repeat this for all the mailboxes you want to keep safe. 
4. Quit Mail.
5. Delete folder: ~/Library/Mail/
6. Delete ~/Library/Containers/com.apple.mail
7. Launch Apple Mail
8. Re-Enable iCloud Mail, reverse step 1.
9. Import each mailbox that was deleted in step 4. Inside Apple Mail Click "File" then "import mailbox." Select the folder of all your mailboxes that you saved and import. 
10. Re-enabled rules and junk mail filter (if you want).

You're welcome. 
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