The Send from Outlook feature is available on most record types, including:
Case
Cash Sales
Check
Company
Contact
Credit Memo
Customer
Customer Deposit
Customer Payment
Estimate
Expense Report
Invoice
item Fulfillment
Item Receipt
Job
Lead
Opportunity
Other Name
Partner
Prospect
Purchase Order
Return Authorization
Sales Order
Vendor
Vendor Bill
Vendor Credit
Vendor Return Authorization
Email using CloudExtend button will appear in NetSuite user interface for record types where emailing is supported.
On clicking this option, you will see an email compose window open in NetSuite which would have the Send from Outlook option.
Learn more about Send from Outlook feature.