Thouxanbanrese if the maid mail includes an item or zeni it will be sent back. if it's just words then it's not important and will be deleted.
All mails have a set shelf life. If they are not read and deleted by the recipient by the time runs out the message will automatically be deleted or sent back to the sender. If you wish to prevent this from happening you can check the "lock" option to stop the shelf life decreasing.