Supporting only ASCII (which essentially means English-only) in this day and age is a form of cultural imperialism. Tor needs users in every country, in every language, to be fully effective and available. So even if we speak only English on this listserv, it would seem inappropiate to fault people for using standards-compliant unicode-capable mailers, or ask them to override default behaviour of their software to support apps on legacy proprietary operating systems.
Source code is available for other mail packages for Solaris, as well as packages like GNU recode. If you choose not to use one, then in the rare case someone posts a base64-encoded message on a subject of interest, you could ask someone to send you the trancoded message.