Saving emails to NetSuite is available to native and custom records and transactions in NetSuite.

Step 1: Go to Settings and select Configure Records Filter.

Step 2: Use the search box or browse the list to find and include the desired Custom Record or Custom Transaction in the lookup.

See other supported record types here for attaching emails.

Once done you will be able to see related custom records or transactions to current email in Available records. You can also search for any custom transaction in Global search. You will be able to associate emails to Custom transactions and also enable Thread or Email Autopilot.