If I send a message through email archive containing the text a=bcd, and the message is orginally sent as 7 char ascii, BasicEmailService will add a header content-transfer-encoding quoted-printable, but it won't actually do the encoding. Thus =bc turns into a 1/4 sign on the Mac. This problem shows up most commonly when Outlook mangles the string href="h. Those characters get dropped, producing broken URLs, e.g. href=ttp://
There are enough paths through this code that it's going to take some work to figure out what to fix. Our copy of 2.8.1 has commented out the setting of the header, in MyMessage, but the same addition of that header occurs several other places in the code.
// if we have a full Content-Type header, set it NOW (after setting the body of the message so that format=flowed is preserved)
if (contentType != null && !contentType.contains("multipart/"))