Here is the list of some helpful links regarding IMAP and POP3 protocols in Exchange 2019:

Enable IMAP4 in Exchange 2019

Enable POP3 in Exchange 2019

POP3 and IMAP4

Public folders in Exchange 2019
Public folders and IMAP

Shared mailboxes in Exchange 2019
Accessing shared and delegated mailboxes

Here is the list of some helpful links regarding IMAP and POP3 protocols in Exchange 2016:

Enable IMAP4 in Exchange 2016

Enable POP3 in Exchange 2016

POP3 and IMAP4

Public folders in Exchange 2016
Public folders and IMAP

Shared mailboxes in Exchange 2016
Accessing shared and delegated mailboxes

Go to your Google Account and on the left navigation panel, choose Security.

On the “Signing in to Google” panel, make sure I that 2-Step Verification is turned on and choose App Passwords. …

Select device and choose the device you’re using. Choose Generate.

Copy the generated password:

Log in to your IMAP, SMTP, POP3 account using your email and the generated password (instead of your email’s password):

Remember – no spaces!

C# code:

using (Imap client = new Imap()) 
{ 
    client.ConnectSSL("imap.gmail.com"); 
    client.UseBestLogin("XXXXX@gmail.com", "kvrcdzlicajaupje"); 

    // ... 

    client.Close(); 
}

VB.NET code:

Using imap As New Imap 
    imap.ConnectSSL("imap.gmail.com") 
    imap.UseBestLogin("XXXXX@gmail.com", "kvrcdzlicajaupje") 

    ' ... 

    imap.Close() 
End Using

Problem

After the installation of the October 8, 2019 — KB4517389 or KB4520003 or KB4519998 or KB4519990 update (depending on OS version), all TLS encrypted data connections to the affected FTP servers fail.

Error you may see is: “TLS session of data connection has not resumed or the session does not match the control connection”

Detailed explanation

In FTP protocol, data connection does not directly authenticate the client.

Client uses control connection to authenticate, then it established data connection using PASV command followed by the STOR (upload) or RETR (download) command.

The server opens a port and waits for the client to connect to it and upload/download files.

An attacker could figure out the port the server listens to, connect to it before the client, and upload a piece of malware.

TLS session resumption prevents this. It acts as a form of authentication. If the TLS session of the data connection matches the session of the control connection, both the client and the server have the guarantee, that the data connection is genuine. Any mismatch in sessions indicates a potential attack.

Ftp.dll library uses .NET’s SslStream that relies on Schannel (Microsoft Secure Channel – a security package that facilitates the use of Secure Sockets Layer (SSL) and/or Transport Layer Security (TLS))

Cause

The KB4517389 addresses the following issue:

“Addresses an issue in security bulletin CVE-2019-1318 that may cause client or server computers that don’t support Extended Master Secret (EMS) RFC 7627 to have increased connection latency and CPU utilization. This issue occurs while performing full Transport Layer Security (TLS) handshakes from devices that don’t support EMS, especially on servers. EMS support has been available for all the supported versions of Windows since calendar year 2015 and is being incrementally enforced by the installation of the October 8, 2019 and later monthly updates.”

It looks like Schannel stared enforcing EMS. If the server runs a TLS stack which is not compatible with this change, the FTP data connection fails.

OpenSSL, which is used by most servers, supports EMS since version 1.1.0 (released 25th August 2016).

Affected Servers

• All FTP servers using OpenSSL older than version 1.1.0
• FileZilla Server (all versions). The latest version uses an insecure/outdated OpenSSL version 1.0.2.11 from 2017.

Solution

Contact the server administrator, explain the situation and and request an upgrade of the FTP server software and of the installed OpenSSL version.

As a temporary workaround, the KB4517389 (or equivalent for non-Windows 10 machines) can be uninstalled.

As a temporary workaround on FileZilla server you can go to “FileZilla Server Interface/Edit/Settings/Ftp over TLS setting” and uncheck “Require TLS resumption on data connection when using FTP over TLS”:

There are two main content types used in all HTML emails: text/plain and text/html MIME types.

You should always include a plain text version of your email that closely matches to the HTML version of your email. Mail.dll will generate (extract) plain text automatically, if you provide HTML text only.

Usually Apple watch displays only the plain text part of your email. In most cases it considers the HTML too complicated (e.g. external images are referenced), so it shows the plain text version instead.

External image is an image that isn’t embedded in the email, using cid: protocol, but loaded from a remote HTTP server using a standard <img src=’…’ /> tag.

You can use ‘text/watch-html’ type to send a limited HTML version of your message to Apple Watch users, resulting in rich text-like messages on Apple Watch devices.

Here’s how to create and add such MIME entity using Mail.dll:

MailBuilder builder = new MailBuilder();
builder.Subject = "Apple Watch Example";
builder.From.Add(new MailBox("alice@example.com"));
builder.To.Add(new MailBox("bob@example.com"));

builder.Html = "This is <strong>HTML<strong> text.";
builder.Text = "Plain text.";

MimeText appleWatchText = new MimeFactory().CreateMimeText();
appleWatchText.ContentType = ContentType.Parse("text/watch-html");
appleWatchText.Text = "This is <strong>Watch HTML<strong> text.";

builder.Alternatives.Add(appleWatchText);

IMail mail = builder.Create();

Please have in mind that apple watch supports limited version of HTML only.

Using the above code will create an email with following content:

Content-Type: multipart/alternative;
 boundary="----=_NextPart_19511516.440335455040"
MIME-Version: 1.0
Date: Tue, 02 Jul 2019 15:38:53 +0200
Message-ID: <d65fcc07-c988-48e1-a466-166e18998d02@mail.dll>
Subject: Apple Watch Example
From: <alice@example.com>
To: <bob@example.com>

------=_NextPart_19511516.440335455040
Content-Type: text/plain;
 charset="utf-8"
Content-Transfer-Encoding: 7bit

Plain text.
------=_NextPart_19511516.440335455040
Content-Type: text/html;
 charset="utf-8"
Content-Transfer-Encoding: 7bit

This is <strong>HTML<strong> text.
------=_NextPart_19511516.440335455040
Content-Type: text/watch-html;
 charset="utf-8"
Content-Transfer-Encoding: 7bit

This is <strong>Watch HTML<strong> text.
------=_NextPart_19511516.440335455040--

Entire sample, including sending process:

MailBuilder builder = new MailBuilder();
builder.Subject = "Apple Watch Example";
builder.From.Add(new MailBox("alice@example.com"));
builder.To.Add(new MailBox("bob@example.com"));

builder.Text = "Plain text";
builder.Html = "This is <strong>HTML<strong> text.";

MimeText appleWatchText = new MimeFactory().CreateMimeText();
appleWatchText.ContentType = ContentType.Parse("text/watch-html");
appleWatchText.Text = "This is <strong>Watch HTML<strong> text.";

builder.Alternatives.Add(appleWatchText);

IMail mail = builder.Create();

// Send the message
using (Smtp smtp = new Smtp())
{
    smtp.Connect("server.example.com");   // or ConnectSSL for SSL
    smtp.UseBestLogin("user", "password"); // remove if not needed

    smtp.SendMessage(mail);

    smtp.Close();
}