: 201.238.231.100 does not like recipient. Remote host said: 550-Callback setup failed while verifying 550-Called: 200.55.206.67 550-Sent: MAIL FROM:<> 550-Response: 530 authentication required 550-The initial connection, or a HELO or MAIL FROM:<> command was 550-rejected. Refusing MAIL FROM:<> does not help fight spam, disregards 550-RFC requirements, and stops you from receiving standard bounce 550-messages. This host does not accept mail from domains whose servers 550-refuse bounces. 550 Sender verify failed Giving up on 201.238.231.100.