Schau die die Admin Hilfe zum Thema Passthru an. Dort steht, dass es so nicht geht, ausser der User verwendet das Notes-Protokoll.
Aus "Passthru servers and hunt groups":
Internet protocols such as HTTP, IMAP, and LDAP cannot use a Domino passthru server to communicate with a destination server.
Aus "Using a Domino passthru server as a proxy ":
You can set up a Domino passthru server as an application proxy for NRPC. A passthru server provides all levels of Notes and Domino security while allowing clients who use dissimilar protocols to communicate through a single Domino server. The application proxy does not allow Internet protocols -- for example, HTTP, IMAP, and LDAP -- to use a Domino passthru server to communicate, however. For Internet protocols, you can use an HTTP proxy with the HTTP Connect method to act as a circuit-level proxy.
A Notes client or Domino server can also be a proxy client and interoperate with either passthru (NRPC protocol only) or as a SOCKS or HTTP tunnel client (for NRPC, POP3, LDAP, IMAP, and SMTP protocols). You set this up in the Proxy setting in the client Location document.
Andreas