SES error missing final '@domain'

Question

I am using python lib bototo send email through SES. And when I tried to send more than 30 (but less than 50, so the limit is not exceeded) recipients, I received an error message:

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

My to_addressesis an empty list, all recipients are in the list bcc_addresses.

What does it mean? Each recipient address is valid and the sender address is verified and valid.