Just because your iPhone is a mobile device doesn't mean it is not also susceptible to junk e-mail. Unfortunately, you cannot use the settings in an iPhone to block junk e-mail messages from reaching your in-box. This is because the Mail application is an e-mail client and not an e-mail service. Therefore, to block junk e-mail from getting to your iPhone, you have to activate the Spam folder, service or filter in your online e-mail account. Although different e-mail service providers have different names for their spam filter, they all share a similar method to activate it.
Sign in to your e-mail account on your iPhone, then click "Options," "Settings" or "Mail Options" in the upper-right corner of the window. If a drop-down list appears, select "Mail Options," or "Mail Settings."
Select "Filters," "Spam," "Junk e-mail" or any combination of those words to open the Spam filter for your e-mail account.
Select the option to block specific e-mail addresses, or let the Spam filter block any e-mail address it suspects is junk e-mail.
Click the "Save" button to save the changes. Click "Yes" on the confirmation window if one appears. The junk e-mail folder is also activated on your iPhone because the Mail application on your iPhone mirrors the settings you have in your online e-mail account.