Fix SMTP forward path formatting
We were encoding mailbox names using RFC 2074 MIME header encoding to handle multi-byte UTF-8 chars, but this is explicitly forbidden by that RFC, so don't do that. Rather, assume by RFC 5322 that we can just pass them through -- that only works for SMTP connections with appropriate UTF-8 support, which we don't support yet, but at this is at least less broken than trying to MIME encode it.
See #336 (closed)