Apple Mail Message/Mailbox Movement Limit

In Mac OS X 10.5 Leopard, the built-in Mail client has an undocumented limit to the number of messages (399) that can be moved successfully from one mailbox to another mailbox.

Exceeding this limit may result in the appearance the messages were not moved.

In Mail, there is a limit of 399 messages or fewer that can be moved from mailbox to mailbox at any one time, i.e. from the Sent mailbox to the Trash mailbox.

Attempting to move 400 or more messages at a time leaves copies in both the source and destination folders resulting in duplicate copies.

Mail may copy the messages from the source mailbox to the destination, but the transaction of removing the messages from the source folder never completes, hence the messages still appear in the source mailbox. Repeatedly attempting to move the same messages will result in duplicate email messages in the destination mailbox(es). 

The 399 message limit does not apply to deleting messages from the Trash folder. Any number of messages may be deleted from here. As this process is not specifically moving messages from one mailbox to another, but deleting messages which is a different type of transaction that does not appear to have this bug.