I have transferred the Connector to a new server. Mail is not delivered. The field for SMTP server choice is empty and I cannot choose another SMTP server.

The connector configuration is stored in AD. You can change the path to the SMTP server using the ADSI edit utility.

First create a new connector and choose in it in the proper SMTP server.

Then launch ADSIEdit. Connect to the Configuration context menu. Open Configuration-> Services-> Microsoft Exchange-> -> Administrative Groups-> First Administrative Group-> Routing Groups-> First Routing Group-> Native POP3 Connector. Right-click this and choose properties.

Copy the parameter msExchRoutingMasterDN of the new connector into the old connector with the configured mailboxes. The new connector may now be removed.