This is a bit sketchy here.  Specifically, you are round-tripping the data through utf-8 (MailParser._write creates a Buffer from this string and converts it back into a Uint8Array using utf-8).  This does the right thing as long as the message is 7-bit safe.  Which e-mails generally are, knowledge no doubt seared into your mind by the horrors you are now privy too.

Where was I?  Oh right, this needs a comment as penance for this / to make it clear to people that it's safe-ish.  My main concern is also that we're generating a fair amount of garbage when we do this that it would be nice to avoid.  So a little mention of that too.

We can probably clean this up a bit when we switch over MIME parsers.  (I expect we'll repurpose bug 918196 for that.)