Thanks for the suggestion. Rolling my own SMTP connection in that way seemed like too much work, so I ended up instead implementing the encoding specified by RFC 3461, and writing a little argument quoter (similar to Aristotle's) to let me invoke sendmail with a 2-arg open. Here is (roughly) the encoder:

sub _quote_envid {
  local $_ = shift;

  # RFC 3461: max 100 chars, pass through printable ASCII characters e
+xcluding
  # [ =+], encode others to '+XX' as uc(hex(ord($char)))
  s{([^\x21-\x7e]|[=+]))}{sprintf "+%02X", ord($1)}ge;

  # we're usually encoding an email address, so trim long strings from
+ front
  # to preserve the domain
  s/^(\+..|.)// while 99 < length;

  $_;
}
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Hugo