From DreamHost

Overview

When an email has been sitting in your Inbox beyond a certain number of days or your Inbox exceeds a certain number of messages, some letters will be moved to a default mailbox named ".old-messages". The exact number of days and number of messages can be configured by individual users by visiting http://mailboxes.yourdomainname.xxx (replacing yourdomainname with your domain name and xxx with com, or org, or whatever). After logging in (with your email username/password), you can modify these settings.

By default the number of days a message can remain in the Inbox (whether it has been read or not) is 45 days. After that point the messages are moved into the .old-messages folder and an email is sent notifying you of this fact. This email contains some links to the wiki search page that claim to be helpful but are, in fact, mostly just confusing. If you follow the first link, select the third result and scroll down to the third sub heading you will arrive at the intended section. Alternatively, you can follow this link. There you will find a paragraph titled, "How do I access other mail folders on the server besides my inbox?"

Accessing Your .old-messages Folder

Perhaps the simplest way to access messages stored in the .old-messages folder is via webmail. Log into your webmail account (webmail.yourdomainname.xxx) and click on the 'folders' link. Towards the bottom there is a block titled "Unsubscribe/Subscribe" and in a form field you will see an option called "INBOX.old-messages". Highlight this option and click the "Suscribe" button. A link to "old-messages" will appear on the left navigation menu and you can use it to browse to those messages.