The "to" keyword documentation seemed incorrect.
Alternatives are presented with "|".
* from - Use "from:nickname" or "from:nickname@domain.tld" to search for posts from a specific author.
-* to - Use "from:nickname" or "from:nickname@domain.tld" to search for posts with the given contact as receiver.
+* to - Use "to:nickname" or "to:nickname@domain.tld" to search for posts with the given contact as receiver.
* group - Use "group:nickname" or "group:nickname@domain.tld" to search for group post of the given group.
* application | relay - Use "application:nickname" or "application:nickname@domain.tld" to search for posts that had been reshared by the given relay application.
* server - Use "server:hostname" to search for posts from a specific server. In the case of group postings, the search text contains both the hostname of the group server and the author's hostname.