Issue: Zoho Mail MCP connector (https://mail-organization-930963415.zohomcp.com/mcp/.../message) — OAuth authorization completes successfully in-browser ("Authentication successful"), but the issued token is rejected when the MCP client reconnects to the endpoint.

Details observed:
- OAuth discovery docs (/.well-known/oauth-protected-resource, /.well-known/oauth-authorization-server) are valid and correctly scoped (ZohoMail.*, PKCE S256, dynamic client registration all present).
- With SSE transport pointed at the /message endpoint: server returns HTTP 405 on connect (endpoint appears to only accept POST, not GET for an SSE stream).
- With HTTP (Streamable HTTP) transport, matching the endpoint's actual design: OAuth flow completes, but the client is told the freshly-issued token is rejected on reconnect ("step-up auth state" was noted as preserved across the retry in debug logs).
- Endpoint TLS/DNS/reachability are all fine — this isn't a network issue.

Ask Zoho: whether their MCP gateway requires an additional step-up verification after the standard OAuth authorization_code exchange, and if so, whether that's documented/supported by standard MCP OAuth clients.