Over at Codestore there are code examples for sending HTML mail via either Java (all platforms) or LotusScript (win32 only).
See Three Part Java example (and sample db), or LotusScript (and sample) version.

Both these samples write to port 25 to manually create an smtp message. Seems like overkill - but of you need to do this and don't want to just use the java mail interface, be my guest.