According to the RFC 6120, section 4.7.2. to the response to the initial stream open response must include to attribute if client provided from attribute in the stream open:

For response stream headers in client-to-server communication, if the client included a 'from' attribute in the initial stream header then the server MUST include a 'to' attribute in the response stream header and MUST set its value to the bare JID specified in the 'from' attribute of the initial stream header. If the client did not include a 'from' attribute in the initial stream header then the server MUST NOT include a 'to' attribute in the response stream header.

Previously RFC 3920 had following requirement:

• to -- The 'to' attribute SHOULD be used only in the XML stream header from the initiating entity to the receiving entity, and MUST be set to a hostname serviced by the receiving entity. There SHOULD NOT be a 'to' attribute set in the XML stream header by which the receiving entity replies to the initiating entity; however, if a 'to' attribute is included, it SHOULD be silently ignored by the initiating entity.

We should adjust behavior of Tigase XMPP Server to match RFC 6120 as it replaces RFC3920 (add to in the response if from was sent in the stream open request.