When both letters and orations of a late antique author are extant, it is very important to distinguish the information they convey. In the past scholars have regarded all the production of an author uniformly, assuming that letters were public documents in the same way as speeches were. Not all letters, however, were public texts and it is necessary to consider each time their tone, characteristics, and audience. In the case of Libanius, scholars have accused this sophist of lack of consistency and duplicitous behavior when he praised or addressed in a neutral way some people whom he attacked violently in his speeches. Letters and orations belong in two different genres so that it is useless to request a great deal of consistency. While the speeches reveal a sophist very critical of his students and show Libanius as a traditional exponent of paganism who upheld the Olympic gods, the letters point to his caring attitude toward his pupils, the close relationship he had with some Christians, and the disappearance of the gods, except Zeus, from his late epistolary messages. Ancient letters were vehicles for friendship. A minimum of politeness was written into their initial and concluding expressions, and aggressive comments were generally ruled out. Yet politeness was a straightjacket so that sometimes one can perceive in a letter hostile or cold behavior that might come fully to the light in a speech. Some apparently innocuous letters of Libanius already reveal that he felt threatened by some people whom he abused in his speeches. An interpreter thus must examine carefully the text of a letter trying to go beyond polite, conventional expressions. In this paper I intend to examine a group of letters that concern Helpidius 4, a Christian governor who in 363 married his daughter Prisca to a relative of Libanius, Bassianus 2. Two families were thus happily joined. All the sources regard the governor as a good, pious, and humane administrator. The sophist, however, attacked him violently around 365 in Or. 37. He accused Helpidius of prostituting himself when he was young, of running to Rome at the bidding of a certain senator who was his lover, and of having a concubine while living with his devout wife. These seemingly made up allegations were inspired by the rhetoric of psogos, a violent invective that appealed to a public that regarded speeches as a form of spectacle. But is the sophist's malevolent attitude absent from the correspondence? Veiled polemic and frigid remarks are hidden under politeness. Genre, and not inconsistent behavior, is a fundamental key to understand the different treatment.