New e-mail server, all my old e-mails disappeared, How do I get them back?

Question

I changed my e-mail provider from Comcast to Verizon and when I updated the accounts in Mail --- all my inbox messages to the Comcast account disappeared. Is it possible to get them back or at least see them?

Answer 1

What type of accounts are these (POP, IMAP)?
Mail stores all mail in ~/Library/Mail/. For each account, there is a folder whose name starts with the account type (POP, IMAP, Mac), followed by the account username, and the incoming mail server. This is where all account-specific mailboxes are stored.
You should have created a new account in Mail, rather than changing the settings of the old one. I've seen people reporting that doing what you did caused the old account folder to be wiped out (and all the mail stored within it to be lost) without warning. This would be a serious bug, but I've been unable to reproduce it. It could very well be that the old account folder is still there, yet inaccessible.
Take a look at the contents of the ~/Library/Mail/ folder. Do you see account folders for both the old and the new account there?
Note: For those not familiarized with the ~/ notation, it refers to the user's home folder. You can easily locate any of the folders referred to in this post by copying the file path here, doing Go > Go to Folder in the Finder, and pasting the file path there.