Wojciech we had a chat about something similar not long ago. What is your opinion on this?

Actually Tigase adhere to the specification. You linked to the old one, and current RFC6121: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, section 8. Server Rules for Processing XML Stanzas, 8.5.3. localpart@domainpart/resourcepart, 8.5.3.2. No Resource Matches states:

8.5.3.2.1. Message

For a message stanza of type "normal", "groupchat", or "headline", the server MUST either (a) silently ignore the stanza or (b) return an error stanza to the sender, which SHOULD be .

For a message stanza of type chat

• If there is no available or connected resource, the server MUST either (a) store the message offline for later delivery or (b) return an error stanza to the sender, which SHOULD be .

• If all of the available resources have a negative presence priority then the server SHOULD (a) store the message offline for later delivery or (b) return a stanza error to the sender, which SHOULD be .

• If there is one available resource with a non-negative presence priority then the server MUST deliver the message to that resource.

• If there is more than one resource with a non-negative presence priority then the server MUST either (a) deliver the message to the "most available" resource or resources (according to the server's implementation-specific algorithm, e.g., treating the resource or resources with the highest presence priority as "most available") or (b) deliver the message to all of the non-negative resources that have opted in to receive chat messages.