I've run across this as well, and contacted my email provider. This explanation tells why we get the crossed out messages, as we don't have all the options on our Pre IMAP client that other systems have.
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Many IMAP clients allow you to configure them to handle deletions is one of several manners. For example, Thunderbird gives you three choices:
* Remove immediately -- a deleted message is removed from the message list display in the client. It is still on the Fastmail IMAP server (with a line through it, showing that a client requested deletion). There is no duplicate message created.
* Mark it as deleted -- a deleted message is marked as deleted (with a line through the message) in the client mailbox listing. It is still on the Fastmail IMAP server (with a line through it, showing that a client requested deletion). There is no duplicate message created.
* Move it to the Trash folder -- a deleted message is marked as deleted (with a line through the message) in the client mailbox listing. It is still on the Fastmail IMAP server (with a line through it, showing that a client requested deletion). A copy of the original message is placed in the Trash folder (and is not marked for deletion unless the user deletes this copy from the Trash folder).
There is often a method provided in the client to clean up those messages marked for deletion, such as the following in Thunderbird:
* Clean up (Expunge) Inbox on exit -- This permanently removes the messages marked for deletion (with a line though the message) from the Inbox on both the client and the Fastmail server. It is executed when exiting the client application. The Fastmail web interface also has an Action command to Purge IMAP deleted messages.
The reason that IMAP has these various message deletion features is to allow safe and flexible message deletion when using various client connections. For example, let's say you use both a mobile email client and a desktop email client. It might be handy to delete some messages on the mobile client, but not remove them from the server permanently until you have seen them using the desktop client. You might not even use a Trash folder.
If you don't move deleted message to Trash and don't use the IMAP marked for deletion (with a line) features, then if you make a mistake you can instantly delete one or more messages permanently.