Mandrill returns reject_reason: 'invalid-sender'

I have a node.js application and I want to use Mandrill to send transactional emails. However, when I send, I get "reject_reason: 'invalid-sender'". Here's some logging to show what's happening:

MandrillMail::send sent 
 { 
  from: 'counselor@nosend.com',
  to: [ { type: 'to', email: 'toddrun@nosend.com' } ],
  text: 'Sample Message',
  html: '<p>Sample Message</p>',
  subject: 'Regarding your account with Zev J' 
 }
and got back 
 [ { 
   email: 'toddrun@nosend.com',
   status: 'rejected',
   _id: '0ef9980ab50d448cb8bc1fcec2d614fc',
   reject_reason: 'invalid-sender' 
 } ]

Note that in the actual log file, the "nosend" values are the real domains, and all the email addresses are real inboxes that I can send test messages to via Gmail.

So, what's causing the 'invalid-sender'? Thanks in advance!