By default, Domino Web Access determines the character set for e-mail being sent, based on settings in the Configuration Settings document (MIME - Settings by Character Set Groups tab). When GuessMIMEBodyLang is set to 0 (default), and the "Use UTF-8 for outout" setting in Server document (Internet Protocols - Domino Web Engine tab) is turned on, Unicode is used as the character set group.
When this is set to 1, Domino Web Access guesses the language of the mail body and uses that language for the character set group.