In Ezra, 4:8, a person called Shimshai is mentioned:
רְחוּם בְּעֵל-טְעֵם, וְשִׁמְשַׁי סָפְרָא, כְּתַבוּ אִגְּרָה חֲדָה, עַל-יְרוּשְׁלֶם--לְאַרְתַּחְשַׁשְׂתְּא מַלְכָּא, כְּנֵמָא.
Rehum the commander and Shimshai the scribe wrote a letter against Jerusalem to Artaxerxes the king in this sort
Rashi says that Shimshai is noone else but Haman's son:
ושמשי ספרא - ושמשי היה סופר הוא שמשי בנו של המן
Where is Shimshai mentioned as Haman's son? He isn't mentioned among
עשרת בני המן the ten sons of Haman that were hanged... So how does Rashi know that?