In order to improve performance system-wide as well as for individual users on the new UTCS IMAP server, we have changed the default namespace while also specifying additional namespaces to support the most common email file layouts of CS users¹. This means that if you are currently using the old IMAP server (mailbox.cs.utexas.edu) without an explicit namespace you will need to make changes when switching to the new IMAP server (mailbox2.cs.utexas.edu). If you already have an explicit namespace specified on your client email software, you are not required to make any changes when switching to the new server.  The following table describes what namespace you should use based on the directory your mail is stored in on the server. The "empty" entries reflect the default namespace for each server, but you can also set an explicit value if you prefer (mail/ is the recommended setting these days).

When you switch to the new server (or when webmail switches to the new server on 8/16/2017), the difference in the default namespace may make it look like your email folders (other than your primary "Inbox") have disappeared when really it's that your email client software and/or the server are just looking in the wrong place for them.  There are multiple ways to solve this situation, listed here in order of preference:

1. If you haven't specified a namespace / prefix in the past, then the files that correspond to your mail folder likely live in your home directory ($HOME). Because the IMAP server tries to search this directory and all subdirectories to find your email, this can incur significant processing load and time (in rough proportion to your disk space usage). In order to see better performance, the preferred solution is to isolate your mail files by moving them into a subdirectory, ideally $HOME/mail (with the exception of $HOME/mailbox, which the system specifically expects to find there). Because the new IMAP server defaults to looking in the $HOME/mail directory, you do not need to explicitly set a namespace / prefix. If your mail client allows setting the prefix and you wish to explicitly set it, you should use "mail/" (without the quotes).
2. If you decide that you don't want to move your mail files (whether because you have so few files that you don't see a significant performance impact, you're uncomfortable with manually moving files, or you're just not ready to do so yet) and your email client allows you to set a namespace / prefix, you should explicitly set it to "home/" (without the quotes). This tells the new IMAP server to look in your $HOME directory for mail files, essentially mimicking the behavior of the old server.

If you'd rather avoid the "where did all my email go" moment and your email client supports explicitly setting the namespace / prefix, you can go ahead and move your mail files and/or specify the appropriate namespace / prefix before changing the name of the IMAP (sometimes labelled "incoming") server.  If you are using webmail, you can now set a namespace (labelled "Folder Path" in webmail) under Options->Folder Preferences.

1. The IMAP standard implements the concept of IMAP Namespaces, which allows servers to specify the location of mail files within a user's account. Most (but not all) mail software can then be configured to use these namespaces to properly match the layout of the user's files. While namespace is the official term used in the IMAP standard, mail software may variously refer to this as "IMAP Mail Prefix", "IMAP Server Directory", "Folder Path", or something similar. While IMAP servers provide a default namespace to use when the client doesn't explicitly set one, not all server software has mapped the default namespace to the same mail file layout, and over time this has led to users having different layouts.