When receiving attachments in the mail one or more of the attached items cannot be retrieved from the mail. In the cases I have observed the items turn up to be still in the inventory of the sender.

When receiving multiple mails in the mailbox upon activating a mailbox an animation associated with receiving loot is displayed for all the items that are attached to all the mails in the mailbox. The items do not wind up in the inventory of the character though, but are still present in the mails from where they can be retreived normally, except for the items displayed but that cannot be retrieved due to the bug described above.

When receiving attachments in the mail one or more of the attached items cannot be retrieved from the mail. In the cases I have observed the items turn up to be still in the inventory of the sender.

A work-around is to send an empty mail to the recipient first, followed by one or more mails with attachments.
This is the same work-around to solve the recipient unknown bug that sometimes occurs when sending a mail with attachments.