Depending on your Preview setting for mail - the number of preview lines shown for a received message dictates how much is downloaded initially with each new message, but another preview line for text does not amount to much data. When you open a received message, the remainder of the message is downloaded. Depending on the size of an included attachment, the attachment may or may not be downloaded when opening the message. If an included attachment is not downloaded when opening the message, you must select the attachment to download the attachment from the server.