High water mark notification

It's a real message but it is coming from your ISP and not Outlook. This mean you'll have to cleanup the web based mailbox provided by your ISP and not the Outlook mailbox.

There are a couple of reasons why this could happen. The 2 most common are:

1. You have Outlook configured to leave a copy on the server.
2. Your ISP filters Junk Email for you

You have Outlook configured to leave a copy on the server

Up until Outlook 2007, this option is disabled by default. As of Outlook 2010, this is set at 14 days.

In this case you'll have to manually cleanup every now and then or reconfigure the option to either not leave a copy on the server or only keep a copy on the server for x couple of days.

You can reconfigure this setting by going to the account settings of your POP3 mail account and press More Settings...-> tab Advanced

Your ISP filters Junk Email for you

While this is a good thing on its own, it could have the nasty side effect of filling up your mailbox without noticing if they move the spam messages to a separate folder.

A POP3 account is only aware of the Inbox folder so the Junk Email folder will never get downloaded and cannot be emptied by Outlook.

There are 4 common ways to work around it:

1. Periodically clean it manually
2. See if you can change the handling of Junk Email from moving it to a specific folder to marking the subject instead like for instance with *****SPAM*****. You can then easily junk these emails in Outlook by creating a message rule. (Recommended)
3. See if your ISP provides an automated maintenance option (many do not). You can then set it to automatically delete items in the Junk Email folder which are older than x days. It's a bit like AutoArchive in that sense.
4. Disable the Junk Email Filter. (not recommended)