SES error missing final '@domain'

I'm using python lib boto for sending emails via SES. And when I tried to send to more than 30(but less than 50, so limit is not exceeded) recipients, I got an error:

  <Error>
    <Type>Sender</Type>
    <Code>InvalidParameterValue</Code>
    <Message>Missing final '@domain'</Message>
  </Error>

My to_addresses is empty list, all recipients are in bcc_addresses list.

What does it mean? Every recipient's address is valid and sender address is verified and valid.