Subject: Base64-encoded message/rfc822 attachment
From: sender@test.invalid
To: recipient@test.invalid
Message-ID: <outer.msg.333880@test.invalid>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="=_outer_boundary"

--=_outer_boundary
Content-Type: text/plain

Outer body.

--=_outer_boundary
Content-Type: message/rfc822
Content-Transfer-Encoding: base64

U3ViamVjdDogSGkKRnJvbTogYWxpY2VAbWltZS50ZXN0LmludmFsaWQKVG86IGJvYkBtaW1lLnRlc3QuaW52YWxpZApNZXNzYWdlLUlEOiA8aW5uZXIubXNnLjMzMzg4MEB0ZXN0LmludmFsaWQ+CkNvbnRlbnQtVHlwZTogdGV4dC9wbGFpbjsgY2hhcnNldD11dGYtOAoKSGVsbG8uCg==

--=_outer_boundary--