Outlook login box won't go away

We have a Windows 2008R2 SBS box that used to run Exchange. We have since moved to Hosted Exchange several years ago. Just in the last 6 months, we have been getting login popups when we are in Outlook asking us to enter our username and password. It says "Microsoft Outlook. Connecting to pfmbx01@mex06.emailsrvr.com."
The problem will only happen with a machine that is connected to our local network. If we try to connect the user on a machine outside of our building, the issue will not occur. The second part of the connection; "mex06.emailsrvr.com" is an address for Rackspace which we use but they say they cannot help us because the issue does not occur outside of our network. Secondly, the user "pfmbx01" is not a user we have.
Outlook will connect properly with the users username and password to autodiscover and work fine but we still get this other popup that comes up several times a day. It doesn't change the connectivity of Outlook; in fact, Outlook works fine when you push cancel or if you put your email username and password in the login box. It doesn't seem to matter.
We have worked many different angles on this problem but cannot seem to find a solution.
This one is a difficult one but an issue that is causing lots of frustration.
Thomas AnthonySecurity AdministratorAsked:
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JonathanSpitfireSenior Solutions EngineerCommented:
Hi TcAnthony,

It is peculiar that this is popping up all of a sudden since you state that you have been hosted for years...but I'll have a go at it.

Make sure that the Credential Manager is clear of any (possible) bad cached credentials.

Off the cuff, it sounds like a UPN or username format issue....I've fought that before. If the format expected is contoso\username, but the username is entered as username@contoso.com, it is not unusual for the login to not be passed properly, and then pop up again as you describe.

Have you run through the tests in the RCA? (testconnectivity.microsoft.com)? Notice that there is a client tab there that will allow you to run the same tests on premise....

Guy LidbetterCommented:
Hi TcAnthony...

These kind of issues are usually related to Shared Mailboxes or Public folder access....
Judging by the name... I'm willing to put money on the fact that pfmbx01 is a Public Folder.
This is backed up by the fact that cancelling the popup still allows user mailbox access.

Public folder connections are different in the fact they are a direct RPC/TCP connection to the Mailbox Server and not via the CAS.

I'm guessing you either have a permissions issue on the public folder stated or there is a firewall issue blocking RPC ports to the mailbox server hosting the PF database.

Either way... you need to speak to your provider to assist in finding where the blocker is.
Thomas AnthonySecurity AdministratorAuthor Commented:
I'm going the public folder direction first. I apologize for not adding that this issue started after Rackspace upgraded us to Exchange 2013. The odd thing again is it only happens on two users, not the rest (12 others internally).
We have no public folders that I know of. What should I look at?
We have talked to Rackspace and they are at a loss as well.
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Thomas AnthonySecurity AdministratorAuthor Commented:
I have cleared out the credential manager for both users, BTW.
We have tried adding the domain to the username but the fact it is asking for the pfmbx01 makes we wonder about the public mailbox as well judging by what I have looked up.
Guy LidbetterCommented:
Since this is EX13... the PF hierarchy mailbox may have been created (pfmbx01@mex06.emailsrvr.com), or they did not migrate\decommission 2010 properly and you're left with remnants.
If you have NO public folders... Get Rackspace to run  this against your database

Get-MailboxDatabase <DatabaseName> | select Name, PublicFolderDatabase

Open in new window

It should be $Null.

But I would suspect that all users would be affected.

Is there anything different about those users? i.e. a different Outlook version? Different site location etc...

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Thomas AnthonySecurity AdministratorAuthor Commented:
If I completely remove Outlook 2010 and install 2013, our issues go away. Everything is the same other than version.
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