If your users have an Office 365 subscription they will benefit from the latest feature sets from Microsoft including task pane pinning. Educate your users on how to use task pane pinning to help drive adoption.
When a user attaches an Outlook email to NetSuite they are required to stay on the active email until the attach process is complete or risk that the email will not attach.
Users with supported versions of Outlook can enable task pane pinning which will allow them to navigate to other emails even while the attach process is underway.
Pinnable task panes are currently available to Office 365 subscribers using Outlook 2016 or later on Windows (build 7668.2000 or later for users in the Current or Office Insider Channels, build 7900.xxxx or later for users in Deferred channels), Outlook 2016 or later on Mac (version 16.13.503 or later), and Outlook on the web.
See this 1 minute video for more information.