Sometimes email messages that have already been read and still in your inbox don't appear on your iPhone. This is due to the fact that the phone configuration is set to display only the most recent ones. To change your iPhone settings, follow this simple guide.
Steps
Method 1 of 2: Check Archived Emails
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Step 1. Launch the "Mail" application
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Step 2. Tap the "Mailboxes" button
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Step 3. Tap the profile associated with the emails you want to find
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Step 4. Click on the "Archive" option
Not all accounts have an archive.
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Step 5. Find the message
Scroll through the archived messages until you find the one you want.
Method 2 of 2: Change the Sync Settings (iOS6)
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Step 1. Go to 'Settings'
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Step 2. Select the 'Mail, Contacts, Calendars' item
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Step 3. Select your email account, then select the item 'Mail days from sync
'.