To get to this page, to to Mail Connectors → (choose Mail Collector) → ODMR Options. This tab is only available if the Mail Collection Method on the General tab is set to ODMR (ATRN) Mail Collection.
ODMR (On Demand Mail Relay) is an SMTP extension where the client (VPOP3) logs into an ISP's SMTP server and then sends a command (ATRN) to switch from being an SMTP client, to being an SMTP server. So, the ISP's SMTP server becomes the SMTP client and sends waiting messages to the VPOP3 SMTP Server.
To use ODMR/ATRN, your ISP needs to support it. Your ISP will tell you all the appropriate details needed to enter into the boxes on this setting page.