Leading double period from .NET Framework's SmtpClient not decoded

[mrmonday]

When loading an email generated by .NET Framework's SmtpClient, if a line starts with a leading ., it is replaced with ...

When using .NET 7 instead of .NET Framework it produces a .eml without the additional leading ..

Program.cs:

using MimeKit;
using System.Net.Mail;

// Set up a temporary pick up directory
string pickupDirectory = Path.GetFullPath("pickup");

try
{
    Directory.Delete(pickupDirectory, recursive: true);
}
catch
{
}

Directory.CreateDirectory(pickupDirectory);

// Configure SMTP client to use it
var client = new SmtpClient()
{
    DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory,
    PickupDirectoryLocation = pickupDirectory,
};

const string goodBody = "<p>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.bbbbbb</p>";
const string badBody = "<p>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.bbbbbb</p>";

// `goodBody` moves the `.` away from the start of the line, avoiding the encoding issue
string body = badBody;

var message = new MailMessage("[email protected]", "[email protected]", "subject", body)
{
    IsBodyHtml = true,
};
client.Send(message);

// Load the generated email and compare the bodies
string file = Directory.EnumerateFiles(pickupDirectory, "*.eml").FirstOrDefault();

var mimeMessage = MimeMessage.Load(file);
if (mimeMessage.HtmlBody.TrimEnd() != body)
{
    throw new Exception("body does not match");
}

Project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <LangVersion>11</LangVersion>
    <OutputType>Exe</OutputType>
    <TargetFramework>net48</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MimeKit" Version="4.1.0" />
  </ItemGroup>

</Project>

Sample generated email when using badBody.

X-Sender: [email protected]
X-Receiver: [email protected]
MIME-Version: 1.0
From: [email protected]
To: [email protected]
Date: 10 Jul 2023 12:49:26 +0100
Subject: subject
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

<p>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=
..bbbbbb</p>

I could not see an option to change the decode behaviour when loading - sorry if I've overlooked something.

[jstedfast]

The files saved by System.Net.Mail are not in MIME format (they have been pre-escaped for SMTP transport) and so MimeKit's parser does not know to unescape them.

If you want them to be unescaped, you'll need to unescape the stream before you parse it.

Unfortunately, neither MimeKit nor MailKit have a filter for this so you'll have to write your own logic, but it should be fairly simple to do.

Is there a reason you are saving the message using System.Net.Mail and then parsing it using MimeKit?

Can you use MimeMessage.CreateFromMailMessage(mailMessage) instead? Or even better, just generate the message using MimeKit?

[mrmonday]

We have a suite of (legacy) applications which use System.Net.Mail to write mail into a pick up directory.

We had previously used the IIS 6 SMTP Server that ships with Windows to pick these up and forward them to a real mail server, but that was an endless source of weird bugs.

We now have a tool based on MailKit which reads these in and forwards them, which is how we encountered this.

Could you point me in the right direction to build such a filter? If you believe it's something that would be more generally useful I'd be happy to open a PR once it's written.

[jstedfast]

The 2 closest filters would probably be https://github.com/jstedfast/MailKit/blob/master/MailKit/Net/Smtp/SmtpDataFilter.cs and maybe https://github.com/jstedfast/MimeKit/blob/master/MimeKit/IO/Filters/Dos2UnixFilter.cs

The SmtpDataFilter is a filter that adds the extra dot (which is the opposite of what you want to do).

Once you've got your filter written, you'll be able to do something like this:

string file = Directory.EnumerateFiles(pickupDirectory, "*.eml").FirstOrDefault();
using var stream = File.OpenRead (file);
using var filtered = new FIlteredStream (stream);
filtered.Add (new CustomSmtpDataUnescapeFilter ());

var message = MimeMessage.Load (filtered);

[jstedfast]

I opted to add it as a mode of the existing SmtpDataFilter instead and optimized the logic a bit.

[mrmonday]

That looks great, thank you!