If MX record is not found, fallback to A or AAAA
Bumped smtp-connection version and replaced simplesmtp based tests with smtp-server based ones.
Fixed a bug with stream buffering. Uses mail.resolveContent provided by Nodemailer v1.1.
As the change includes a method from Nodemailer 1.1 and not 1.0, then changed the version scheme to use proper semver instead of 0.x.