I'm sorry, I misread your other post and thought you were using MSN mail, instead of MTS. I was up too late.
Do you have Norton Anti-Virus, or another Anti-virus program that automatically checks your mail? If so, the account may be set to be routed through the Norton Servers in Norton AV, but NOT in your mail program. If this could be the case, try opening Norton AV and checking the Email settings. There should be a checked checkbox beside each email account in the Norton list. If the problem account is checked off, UNcheck it. If the account is unchecked, try checking it. Then check your mail. (re-enable the virus-checking later if this works and see if it's ok then)
If this isn't the case, well, I did look up the error at Microsoft. Error 0x800xxx0D means that the mail program couldn't see the mail server at the ISP.
Microsoft says this error is caused by faulty configuration in the account setup, and suggests trying all the things you've already tried.
BUT, maybe you didn't verifiy the Log on Using Secure Password Authentication box or the My Server Requires Authentication boxes on the Servers tab in the Account Properties? Please double-check these to make sure they are the same as the account that works.
Also, on the Advanced tab, make sure the Server ports are the same (probably 25 for SMTP and 110 for POP3), and that the SSL boxes are the same (probably NOT checked).
This article at Microsoft goes over troubleshooting steps, but I read through it and it IS mostly what you've done already.
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q274456
I'm not finding anything else on this problem; if this doesn't fix things, surely somebody else has some ideas? My only other suggestion would be to remove the account totally (again, I know) and enter everything from scratch.
Keep us up to date on how you're making out.