In order to reduce storage space for your @ieee.org account, you may have to export your email data to another third party service, and then purge your email from the @ieee.org service. Please see the below instructions on how to use a free personal @gmail.com account to accomplish this.
In order to complete this process, a few steps are necessary, which will be outlined in detail below, with screenshots. The process includes getting your @ieee.org account ready for exporting; creating and then logging into a personal @gmail.com account; set up the import from the @gmail.com account to access your @ieee.org account; monitor the import for completion; remove the import rule from @gmail.com; set up forwarding on your @ieee.org account to @gmail.com account.
In order to allow @gmail.com to import email from your @ieee.org account a few steps will need to be performed. You will need to log into your @ieee.org account, allow POP access, enable two-factor authentication, and then set up an application specific password.
Once you are logged into your @ieee.org account, you can click the gear icon(1) in the upper right of the window, and choose 'See all settings'(2). You will then go to the 'Forwarding and POP/IMAP' tab(3), and choose the 'Enable POP for all mail'(4) selection, as well as choosing 'Delete IEEE mails copy'(5), then scroll down to the bottom, and choose save. Then verify the Status is updated to the new setting.
You are now going to set up the import from your @ieee.org account into your @gmail.com account. Please note: The folder structure of your @ieee.org email account will not be preserved. The emails will be migrated together into your "All Mail" label on your @gmail.com account. It is strongly recommended that you use Google Takeout to export a copy of your IEEE Mailbox before continuing.
Log into your External (Non-IEEE) @gmail.com account and go to Settings (gear icon in the upper right) and choose 'See all settings'.
Choose the 'Accounts and Import' tab, and scroll down to the 'Check mail from other accounts:' section.
Choose 'Add a mail account'.
Enter your full @ieee.org email address in the next window, and click 'Next'.
The Import selection should already be chosen, so click 'Next'.
On the next screen, please set the options as in the screenshot. In the password field, use the Application Specific Password you created in the previous step. The server is going to be pop.gmail.com, the port is 995, check the box for SSL, Label incoming messages, and Archive incoming messages. Leave the first box unchecked, as Google servers do not support this import option. Click the 'Import' button. On the next screen choose 'No' on sending as this account, as we are only using this for backup purposes.
Your messages are now being imported. You can review this process by going to Settings \ See All Settings \ Accounts and Import tab. It will show you the status of your import. Depending on the volume of emails, this may take a few hours or a few days.
Once the import has completed, you can remove the import rule from Settings \ Accounts and Import tab. This will stop pulling your email from your @ieee.org account.