Important: If Mail unexpectedly quits when viewing very large email attachments, try updating your iPhone or iPod touch to 2.2 or later. If that doesn't solve the issue, see below for additional information and troubleshooting steps.
If this occurs with IMAP or POP account
Restart the iPhone or iPod touch and try to view the attachment again in Mail (Press and hold the Sleep/Wake button until the red slider appears. Slide your ﬁnger across the slider to turn off iPhone. To turn iPhone back on, press and hold the Sleep/Wake until the Apple logo appears.).
If this occurs with a Microsoft Exchange account
To work around this issue, open the affected mail account on your desktop computer and view the attachment on your computer instead. Afterwards, move the message(s) with the large attachments out of the active Inbox in your email account or delete the message(s) if you no longer need it.