_enc_header() checks if a value is ASCII-safe; if not, wraps it in
email.header.Header with utf-8 charset, producing RFC 2047 encoding
(=?utf-8?b?...?=). Applied to Subject and To in _build_mime — covers
both send and reply with Cyrillic subjects or display names.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>