bbottema
Support having different Content-Transfer-Encoding in multipart messages
Hi.
I noticed that when parsing an email from its .eml representation and sending it over to an SMTP server using simple-java-mail the Content-Transfer-Encoding header is changed for some parts of the message.
For example:
This is my .eml file:
From: [email protected]
To: [email protected]
Subject: Example multipart message
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="boundary123"
--boundary123
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Hello,
This is the plain text version of the email.
--boundary123
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
<html>
<body>
<p>Hello,</p>
<p>This is the <b>HTML</b> version of the email.</p>
<p>Notice the non-breaking space: and =C2=A9 symbols.</p>
</body>
</html>
--boundary123--
After parsing it from a file and sending it to the SMTP like this:
var eml = Files.readString("etc etc");
var email = EmailConverter.emlToEmail(eml);
mailer.sendEmail(email);
What I got on the other side is something like this:
Content-Type: multipart/alternative; boundary="----=_Part_0_416639874.1756482293892"
Date: Fri, 29 Aug 2025 12:44:53 -0300 (ART)
From: [email protected]
MIME-Version: 1.0
Message-ID: <mex07fbe.1hqvcchgqwx16@localhost>
Received: from 192.168.250.164 by mailhog.example (MailHog)
id [email protected]; Fri, 29 Aug 2025 15:44:53 +0000
Reply-To: [email protected]
Subject: Example multipart message
To: [email protected]
------=_Part_0_416639874.1756482293892
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Hello,
This is the plain text version of the email.
------=_Part_0_416639874.1756482293892
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: 7bit
<html>
<body>
<p>Hello,</p>
<p>This is the <b>HTML</b> version of the email.</p>
<p>Notice the non-breaking space: and © symbols.</p>
</body>
</html>
------=_Part_0_416639874.1756482293892--
Please note that in the second part of the message, the Content-Transfer-Encoding was changed fro quoted-printable to 7bit. I think that the converter is just taking the first Content-Transfer-Encoding that it sees and applying it to every part that the message has.
Do you think there's a way that having different transfer encodings for each part can be supported?
