I've checked the document and it is said that with the "Send Email" permission, user can send email by click Send An Email on the Activity History related list of a record.
I'm using an user with the "Send Email" permission enabled. The user license is force.com free. I clicked the "Send an Email" button on one of the custom record and got the "Insufficient Privileges" error.
Following are the details form the help document. Thanks!
Available in: All Editions except Database.com
User Permissions Needed
To send emails:
To create an organization-wide address:
“Modify all Data”
To send email to a contact, lead, person account, or other user:
- Click Send An Email on the Activity History related list of a record. Or, from a case, click Send An Email or Reply from the Email related list. This option is available only to organizations using Email-to-Case or On-Demand Email-to-Case.