This article distinguishes between email servers and email programs (email programs are also called email clients or apps). Cornell's email server is Exchange 2010 from Microsoft. Email clients are programs such as Outlook, Mail, and Thunderbird that run on your desktop or mobile phone, and access mail on the mail server. Outlook on the Web is also a mail client, but a web-based one that you access through a web browser. An analogy using the phone system is that the telephone on your desk is a client, and Verizon is the server.

Many conversations with upset users start one of two ways: "Your mail server lost my mail" or "Can you restore my mail?" Let's deal with those first.

"Your mail server lost my mail!"

Actually, that has never been the case in any of the reports we have investigated. The mail server handles the actual delivery of new mail to an Exchange account, but all other changes and actions are made through an email program such as Outlook. It's always an action by one of these programs that makes mail disappear.

"Can you restore my mail?"

Unfortunately, no. We back up the Exchange database as a whole, as a protection against disaster. It's impractical to restore an entire Exchange database to restore a single mailbox. The good news is that Exchange doesn't really delete mail items until 30 days after you've told it to. It first keeps deleted items in a special folder called Deleted Items, (some programs label this as Trash). After the Deleted Items folder is emptied, items are held in a dumpster for 30 days, and you can usually find missing items there. See our Recover Deleted Items article for detailed instructions.

Here are the most common ways to lose email, and what to do about them.

Conversation View [affects all Office 365 clients]

If your messages appear to be out of order, or some messages (especially replies) go missing, you may be in Conversation View. This groups together all messages in a thread (that is, those messages with identical Subjects). 

• Conversation View in Outlook for Windows
• Conversation View in Outlook for Mac
• Conversation View in Outlook on the web

POP [affects older clients and some mobile phones]

POP is an older protocol from the era of small mail servers. It downloads the entire contents of your inbox to your computer. When you check your mail from another computer or device, this can make it look like all the email has disappeared. A common accident here is to start up an old client that you haven't used for a while, and later find that all email is gone from your other clients. If you have switched to Outlook as an email program, but have recently started up an old copy of Eudora (perhaps to find an ancient message you received in the Eudora days), take a look at Eudora to see if it has claimed all of your email for itself. If your program cannot use one of the native Exchange protocols (MAPI, Outlook Anywhere, Exchange Web Services, or ActiveSync), use IMAP (a protocol that doesn't move messages off the server) for the connections.

Rules or Filters [can affect any email client]

Most email programs implement rules for automatically processing mail. Some of them, such as Outlook and Outlook on the Web, can also set up rules on the Exchange server so they can be executed even when that program is not running. (Rules are called client-side or server-side depending on where they run.) Rules can move email items to unintended places, including the trash. Or a rule intended for one type of message can accidentally apply to other types. Rules are also not compatible between different email programs. Don't try to set up a server-side rule in one program and change it in a different one. Don't try to run client-side rules on more than one email program. If you're dealing with missing email, turn off all rules on both client-side and server-side (using Outlook on the Web), and then re-enable them one at a time, checking for proper operation.

Synchronization Errors [affects mobile devices]

Most mobile devices work differently from traditional mail programs. They attempt to synchronize changes made on the device, possibly while out of contact from the network, with the mail on the server. Once in a while, a newly set up mobile device will try to overwrite the email on the server with an empty mailbox. If your mobile device has the option, make sure it is set for server changes to take preference over changes on the device. (The wording of this option will vary from vendor to vendor.)

Compromised Account [can affect any email client]

If a hacker has stolen your NetID and password, sometimes they will delete email to cover their tracks, or to make room for the replies to the 100,000 pieces of spam they just sent in your name. This is rare, but it does happen. If email disappears and there have been other signs of suspicious activity, this is worth investigating. The IT Service Desk can get you in touch with security experts to verify if this has happened to you.

Drag & Drop Errors [can affect any email client]

A frequent culprit for missing folders is the drag error. When intending to select or move a mail item, it can be easy to accidentally drag one folder inside another folder. Some trackpad devices and touch screens can make this type of error quite easy. If you're missing a folder, try expanding other nearby folders to see if the missing item has been dropped in a hidden place.

Shared Mailboxes [can affect any email client]

In any shared mailbox situation, where multiple people access a mailbox at the same time, the number of opportunities for side-effects are multiplied. All of the above scenarios must be investigated for *every* device used by every person who can potentially access the mailbox. Sometimes it is necessary to remove all but one user (and that user from only one location) to stabilize the situation, then add users and devices back one at a time until it is determined which combination causes the problem.