This lookup results in a delay the current connection is to extend to. This may cause a five times longer delay as configured. Several UBE senders try to shorten the SMTP dialog by avoiding unnecessary parts. So keep this in mind. That's why it is filtered from the EHLO response for this connection.
New connection Assume a new incoming connection.
If the delay is zero, the connection is hand over directly to a new spawned MTA process. Holding a connection If the delay is not zero, every command is send to the MTA coprocess and the returned answers are send back. The last line is stored instead of send out. After this, the final line is sent, so a new command can be send by the other side. This may cause your MTA to fork, but there should be an option to prevent this.