Content-Type: multipart/mixed; boundary="boundary" This shouldn't appear. --boundary Content-Type: application/octet-stream Content-Transfer-Encoding: base64 VGhpcyBpc24ndCByZWFsbHkgYW4gYXBwbGljYXRpb24vb2N0ZXQtc3RyZWFtLiA7KQ==' --boundary Content-Type: image/png Content-Transfer-Encoding: base64 TmVpdGhlciBpcyB0aGlzIGFuIGltYWdlL3BuZy4= --boundary Content-Type: multipart/related; boundary="boundary2" --boundary2 Content-Type: text/html This part should be returned. --boundary2-- --boundary Content-Type: text/plain This part shouldn't. --boundary Neither should this part! --boundary--