If you find that your Sent Mail folder isn't showing on your iPhone mail account, follow these steps to view your Sent Mail folder:
1. In your iPhone, go to Settings
2. Click Mail
3. Click on IMAP account
4. Click the Account email address in blue
5. Scroll down to Advanced
6. Click on Drafts.
7. It may show that the Sent Messages is checked, but you will need to check it off again.
8. Now go back and save the changes you made (click Done) for the changes to take effect.