I have a POP account set up in Mail, has been that way for years. Today it isn't working.

I have a POP email account that's been set up in Mail for years. Today it won't send or retrieve email, continually asks for my password but doesn't keep it. My iCloud email is fine. According to an Apple Support document, "Mail: Troubleshooting sending & receiving email" (http://support.apple.com/kb/TS3276), it appears as if it's a problem with my email provider. I've spoken to the email provider and they say it's not a problem on their end. I can log into my email account using the provider's webmail and all is fine there. And, since I'm still receiving my email on my iPad & iPhone, I'm not entirely convinced it is a problem with the email provider.

When I go into Mail Preferences, my password shows blank (even when I re-enter it) and it shows my Outgoing Mail Server as Offline. When I choose Edit SMTP server list and click on the Advanced tab, my password is missing here as well. Re-entering it doesn't help.

I've tried Take All Accounts Offline then Take All Accounts Online and that doesn't work. I would delete the account and set it up again, but I get a warning message that if I delete the account, I lose everything in the account; folders, email, filters, etc. I can't afford to lose the existing emails.

I'm using Mountain Lion 10.8.2 and Mail 6.2; I believe they are both the most recent versions.