I've deleted all messages from my Gmail mailbox using the following code:
Pop3 client = new Pop3();
client.Connect("pop.gmail.com", Pop3.DefaultImplicitSslPort, null, Pop3Security.Implicit);
client.Login("username", "password");
Pop3MessageCollection messages = client.GetMessageList(Pop3ListFields.FullHeaders);
Console.WriteLine("Processsing {0} messages...", messages.Count);
foreach (Pop3MessageInfo msg in messages)
client.Delete(msg.SequenceNumber);
client.Disconnect();
The first execution successfully processes all messages, any next execution finds no message in my POP3 mailbox.
However, when I connect to the Gmail through their web pages, the deleted messages are still visible in my Inbox.