Simple Steps To Fix The iPhone Email Forwarding Problems

Step 1 - Mail application settings

Navigate to the Settings screen on your iPhone and tap the Mail option.
Go to the Threading section and then turn on the toggle button next to the ‘Organize by Thread’ and ‘Complete Thread’ options.
Navigate to the Composing section, tap the Include Attachments with Replies option, and select the Always option.
Close the Mail application and open it again.

Launch the Settings application, tap the General option, and select the Shut Down option.

Turn off the iPhone and then turn it on by pressing the Power button.

Open the Mail application and check if the iPhone email forwarding problems has been resolved by sending a forward test email.

On the Settings screen, tap the Passwords & Accounts option followed by the Accounts option and select your email address.
Tap the Delete Account option followed by the Delete from My iPhone option.
If you have configured multiple email account, perform the same to remove all the email accounts.
Return to the home screen of the iPhone, search for the Mail application, and hold it.
Select the Edit Home Screen option and then tap the X button followed by the Delete option.
Once you tap the Done option, the Mail application will be removed from the iPhone.
Restart the iPhone, launch the App Store application, and type Mail in the search bar.
Choose the Mail application from the search results and tap the Get option.
This will install the Mail application.
Return to the Settings screen, tap Password & Accounts -> Add Account, and select your email service provider.
Provide your email account’s email address and password and then tap the Next option.
Continue with the on-screen instructions.
Finally, tap the Save option at the top of the screen.
Make sure both the Organize by Thread and Complete Thread options are enabled. And then check the iPhone email forwarding problems is continued.