Один из первых же абзацев из доки (правда версии 6) про MimeMessage

A note on RFC 822 and MIME headers

RFC 822 header fields must contain only US-ASCII characters. MIME allows non ASCII characters to be present in certain portions of certain headers, by encoding those characters. RFC 2047 specifies the rules for doing this. The MimeUtility class provided in this package can be used to to achieve this. Callers of the setHeader, addHeader, and addHeaderLine methods are responsible for enforcing the MIME requirements for the specified headers. In addition, these header fields must be folded (wrapped) before being sent if they exceed the line length limitation for the transport (1000 bytes for SMTP). Received headers may have been folded. The application is responsible for folding and unfolding headers as appropriate.


Оно как-то не для реалий России и "когда реализация давно уже выскочила за рамки стандарта" )))

Pcrepairисходники... а что исходники?
Ну как минимум "The application is responsible for folding and unfolding headers as appropriate" и прочие подводные камни