Answer
Truth is a defense, but not by itself. Under Article 361 of the Revised Penal Code, the truth of the imputation leads to acquittal only when it was published with good motives and for justifiable ends (and, for imputations not constituting a crime, truth is admitted mainly where it concerns a public officer's official duties).
Privileged communication is the other key defense. Article 354 exempts from the presumption of malice a private communication made in the performance of a legal, moral, or social duty, and a fair and true report of official proceedings made without comment — though malicious commentary attached to a privileged report remains punishable (Article 362).
Sources & further reading
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