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