UPDATE:
The log showed, that the server returned only two messages, which is behavior of the server and client cannot change this.
Probable cause is configuration of the POP3 access on Gmail; or some kind of Gmail optimization when using another client before (Thunderbird).
The suggested solution is to use IMAP protocol; or use only one POP3 client to access Gmail.
Can you please create VERBOSE communication log and share it here or send it to use (support@rebex.net) for analysis.
It can be created like this:
var pop3 = new Rebex.Net.Pop3();
pop3.Connect(hostname, SslMode.Implicit);
pop3.Login(username, password);
pop3.LogWriter = new Rebex.FileLogWriter(@"c:\data\pop3.log", LogLevel.Verbose);
Pop3MessageCollection emailHeaders = pop3.GetMessageList(Pop3ListFields.FullHeaders);
Assign logger after Login()
call to prevent exposing sensitive data in log.