Currently Being ModeratedMay 8, 2012 2:36 PM (in response to nomeansyes)
Attachments that big will only download if you tap on them. So don't tap on them.
Currently Being ModeratedDec 4, 2012 6:32 PM (in response to AKRBTN)
Akrbtn, that isn't the complete story.
If an e–mail with an attachment is forwarded from iPhone Mail, it will first download the attachment, then send it, so it uses double the bandwidth of the attachment, even though you haven't opened it.
By default, it will also CC you and possibly download it again. So you can end up using 3 times the bandwidth of the attachment.