Even with a spam filter, you can end up getting a lot of e-mail. You probably respond to or delete most of it, but there may be a few that you don't want to (or can't) respond to immediately. You can deal with these in one of two ways.
First, if you know what needs to be done with the message, you can forward it to yourself with a description of what needs to be done in the message header or the text body. On the iPhone or iPod touch, simply open the message in your Inbox, hit the "Reply/Forward" button at the bottom, and select "Forward."
Second, if you need to look at it again but don't know what needs to be done, you can mark it as unread. On the iPod touch, open the message, tap on "Details" and select "Mark As Unread." On the iPhone, open the message and tap on the "Mark As Unread" button.