Here is an example of how you can download all messages from one POP3 server. It takes the various connection settings as arguments.

/// <summary>
/// Example showing:
///  - how to fetch all messages from a POP3 server
/// </summary>
/// <param name="hostname">Hostname of the server. For example: pop3.live.com</param>
/// <param name="port">Host port to connect to. Normally: 110 for plain POP3, 995 for SSL POP3</param>
/// <param name="useSsl">Whether or not to use SSL to connect to server</param>
/// <param name="username">Username of the user on the server</param>
/// <param name="password">Password of the user on the server</param>
/// <returns>All Messages on the POP3 server</returns>
public static List<Message> FetchAllMessages(string hostname, int port, bool useSsl, string username, string password)
{
    // The client disconnects from the server when being disposed
    using(Pop3Client client = new Pop3Client())
    {
        // Connect to the server
        client.Connect(hostname, port, useSsl);

        // Authenticate ourselves towards the server
        client.Authenticate(username, password);

        // Get the number of messages in the inbox
        int messageCount = client.GetMessageCount();

        // We want to download all messages
        List<Message> allMessages = new List<Message>(messageCount);

        // Messages are numbered in the interval: [1, messageCount]
        // Ergo: message numbers are 1-based.
        // Most servers give the latest message the highest number
        for (int i = messageCount; i > 0; i--)
        {
            allMessages.Add(client.GetMessage(i));
        }

        // Now return the fetched messages
        return allMessages;
    }
}

Notice that the for loop starts at one, and goes up to and includes the messageCount number.
This is because POP3 is 1-based. This is the case for all methods taking a message number as an argument.