Sparkpost API method send out email with Content-Transfer-Encoding: quoted-printable format.
In some case, notification email contain multiple Content-Transfer-Encoding: header.
Usually their is a Content-Transfer-Encoding: 8Bit for mail title. Then followed by Content-Transfer-Encoding: quoted-printable (sometimes multiple encoding header) with mail contents. Where, character like =E8 would not be converted and become random code.
Such problem does not happen when mail send out by SMTP. We believe this is a bug in Sparkpost server, not in the Extension:Sparkpost. Support ticket was submitted. Wish Sparkpost could fix this error soon.