From what I can see, the emails sent using the "Send this product to a friend" functionality are the only emails in which the customer can set an arbitrary "From" email address.
I propose that the "From" email is a pre-configured Magento email identity, in keeping with all other usages of the email framework.
This will allow 3rd party email deliverability services such as SendGrid, & Mandrill to deliver the email, as most services require correct SPF/DKIM records for sending domains. I'm sure it would have a positive effect on spam filters as well.
We can set the "Reply-To" mail header to the customer's email address, to allow replies to continue as usual.
I'm happy to create a pull request for this functionality, if desired.
