Solr indexing appends domain to users

[G H]

My users are set up as "admin", "john", etc -- as opposed to john at example.com. When I run test command "/usr/libexec/dovecot/dovecot-lda -d admin < /tmp/mymail.txt", the mail is successfully imported with Solr, Tika (yay). However, results are never found when I search within IMAP. Why? Dovecot is appending the domain name to the user and admin fields of the Solr record ("admin at example.com" instead of just "admin") during index. But during searching, Dovecot is only searching for the user directly, from the Solr logs:
  q=body:"TESTCONTENT"&fq=%2Bbox:7dff0b0d4f84df53f25300003d823887+%2Buser:"admin"

If I rerun the query via the Solr web gui with user:"admin at example.com", the message is found.

How do I disable appending the domain name to the solr index names?

Thanks,
G H