Currently, NetSuite for Outlook does not support saving of mail to NetSuite. See enhancement request#: 257148 NetSuite for Outlook > Ability to save email to a vendor record..
As an alternate solution, edit a vendor record and then add an Other Relationship= Customer.
Here are the steps:
1. Navigate to Lists > Relationships > Vendors.
2. Edit an existing Vendor Record.
3. In the Relationships tab, click the Plus button.
4. In the Type field, select Customer.
5. Click Save.
If someone opens an email in MS Outlook > Click the Sync Manager > Click Search for more.. button > The user should now be able to search for the vendor name under Record Type= Customer.