Hi!

I hit this bug on python 3.4.

For the reference here are two email sources. The first one works while the second one does not. Both were produced using smtplib the only difference being the workaround proposed by Douglas Thor.
I hit this bug when using K-9 mail for reading e-mails.
Relevant github link: https://github.com/k9mail/k-9/issues/1659#issuecomment-416074768

This one works:
---------------

Return-Path: <sender@senderdomain.com>
X-Original-To: receiver@receiverdomain.com
Delivered-To: receiver@receiverdomain.com
Received: from [127.0.0.1] (smtp [127.0.0.1])
        by smtp.receiverdomain.com (Postfix) with ESMTP id 2697F23C0063
        for <receiver@receiverdomain.com>; Mon, 27 Aug 2018 09:20:15 +0300 (EEST)
Subject: Malformed base64
From: sender@senderdomain.com
To: receiver@receiverdomain.com
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
MIME-Version: 1.0
Message-Id: <2697F23C0063@smtp.receiverdomain.com">20180827062015.2697F23C0063@smtp.receiverdomain.com>
Date: Mon, 27 Aug 2018 09:20:15 +0300 (EEST)
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----------------

This one does not:
------------------

Return-Path: <sender@senderdomain.com>
X-Original-To: receiver@receiverdomain.com
Delivered-To: receiver@receiverdomain.com
Received: from [127.0.0.1] (smtp [127.0.0.1])
        by smtp.receiverdomain.com (Postfix) with ESMTP id 0D25523C0063
        for <receiver@receiverdomain.com>; Mon, 27 Aug 2018 09:21:13 +0300 (EEST)
Subject: Malformed base64
From: sender@senderdomain.com
To: receiver@receiverdomain.com
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
MIME-Version: 1.0
Message-Id: <0D25523C0063@smtp.receiverdomain.com">20180827062113.0D25523C0063@smtp.receiverdomain.com>
Date: Mon, 27 Aug 2018 09:21:13 +0300 (EEST)

Q09OVEVOVF9MRU5HVEg6IDEyMzQ1Njc4OTBhYmNkZWYKQ09OVEVOVF9UWVBFOiAxMjM0NTY3ODkwYQ==
YmNkZWYKQ09OVEVYVF9ET0NVTUVOVF9ST09UOiAxMjM0NTY3ODkwYWJjZGVmCkNPTlRFWFRfUFJFRg==
SVg6IDEyMzQ1Njc4OTBhYmNkZWYKRE9DVU1FTlRfUk9PVDogMTIzNDU2Nzg5MGFiY2RlZgpHQVRFVw==
QVlfSU5URVJGQUNFOiAxMjM0NTY3ODkwYWJjZGVmCkhUVFBTOiAxMjM0NTY3ODkwYWJjZGVmCkhUVA==
UF9BQ0NFUFQ6IDEyMzQ1Njc4OTBhYmNkZWYKSFRUUF9BQ0NFUFRfRU5DT0RJTkc6IDEyMzQ1Njc4OQ==
MGFiY2RlZgpIVFRQX0FDQ0VQVF9MQU5HVUFHRTogMTIzNDU2Nzg5MGFiY2RlZgpIVFRQX0NBQ0hFXw==
Q09OVFJPTDogMTIzNDU2Nzg5MGFiY2RlZgpIVFRQX0NPTk5FQ1RJT046IDEyMzQ1Njc4OTBhYmNkZQ==
ZgpIVFRQX0ROVDogMTIzNDU2Nzg5MGFiY2RlZgpIVFRQX0hPU1Q6IDEyMzQ1Njc4OTBhYmNkZWYKSA==
VFRQX09SSUdJTjogMTIzNDU2Nzg5MGFiY2RlZgpIVFRQX1JFRkVSRVI6IDEyMzQ1Njc4OTBhYmNkZQ==
ZgpIVFRQX1VQR1JBREVfSU5TRUNVUkVfUkVRVUVTVFM6IDEyMzQ1Njc4OTBhYmNkZWYKSFRUUF9VUw==
RVJfQUdFTlQ6IDEyMzQ1Njc4OTBhYmNkZWYKUEFUSDogMTIzNDU2Nzg5MGFiY2RlZgpRVUVSWV9TVA==
UklORzogMTIzNDU2Nzg5MGFiY2RlZgpSRURJUkVDVF9IVFRQUzogMTIzNDU2Nzg5MGFiY2RlZgpSRQ==
RElSRUNUX1NTTF9UTFNfU05JOiAxMjM0NTY3ODkwYWJjZGVmClJFRElSRUNUX1NUQVRVUzogMTIzNA==
NTY3ODkwYWJjZGVmClJFRElSRUNUX1VOSVFVRV9JRDogMTIzNDU2Nzg5MGFiY2RlZgpSRURJUkVDVA==
X1VSTDogMTIzNDU2Nzg5MGFiY2RlZgpSRU1PVEVfQUREUjogMTIzNDU2Nzg5MGFiY2RlZgpSRU1PVA==
RV9QT1JUOiAxMjM0NTY3ODkwYWJjZGVmClJFUVVFU1RfTUVUSE9EOiAxMjM0NTY3ODkwYWJjZGVmCg==
UkVRVUVTVF9TQ0hFTUU6IDEyMzQ1Njc4OTBhYmNkZWYKUkVRVUVTVF9VUkk6IDEyMzQ1Njc4OTBhYg==
Y2RlZgpTQ1JJUFRfRklMRU5BTUU6IDEyMzQ1Njc4OTBhYmNkZWYKU0NSSVBUX05BTUU6IDEyMzQ1Ng==
Nzg5MGFiY2RlZgpTRVJWRVJfQUREUjogMTIzNDU2Nzg5MGFiY2RlZgpTRVJWRVJfQURNSU46IDEyMw==
NDU2Nzg5MGFiY2RlZgpTRVJWRVJfTkFNRTogMTIzNDU2Nzg5MGFiY2RlZgpTRVJWRVJfUE9SVDogMQ==
MjM0NTY3ODkwYWJjZGVmClNFUlZFUl9QUk9UT0NPTDogMTIzNDU2Nzg5MGFiY2RlZgpTRVJWRVJfUw==
SUdOQVRVUkU6IDEyMzQ1Njc4OTBhYmNkZWYKU0VSVkVSX1NPRlRXQVJFOiAxMjM0NTY3ODkwYWJjZA==
ZWYKU1NMX1RMU19TTkk6IDEyMzQ1Njc4OTBhYmNkZWYKVU5JUVVFX0lEOiAxMjM0NTY3ODkwYWJjZA==
ZWYK
-----------------