Receiver didn't send an ACK, it got lost, or was delayed. Original packet was delayed or lost in transit. This example has duplicate requests and matching duplicate responses. Server sends a greeting message, beginning with a 220 status code.
Client uses HELO command for introduction.
Server sends an ACK packet, to indicate the message was received. Server sends an ACK packet, to indicate the message was received. Client gives the DATA command, indicating it's ready to start sending the message. Server sends an ACK packet, to indicate the message was received. Client sends and ACK packet, to indicate the status message was received.