Broken pipe usually means the remote end closed the connection unexpectedly.

I took a look at freepops, and it's not immediately clear to me how to set it up.  I don't really have the time to spend on this setting up some random 3rd party software.  So, two options:

1.  Tell me exactly how to set it up to reproduce your problem.

2.  Recompile mailwatch with --enable-debug=yes, kill your panel ('xfce4-panel -x'), restart the panel from a terminal, and capture the output as it tries to connect.

Problem is the assumption that each pop3_recv will always return exactly one line (io up to a "\r\n"). This causes mailwatch to misunderstand what the server is saying, and end up the session prematurely (and i suspect some other bug in the error handling code, because in strace i see an attempt at sending a 
"QUIT" long after the socket is closed).

With the patch below, yahoo POP works:

*** orig/xfce4-mailwatch-plugin-1.0.1/panel-plugin/mailwatch-mailbox-pop3.c	Thu Apr 20 10:24:36 2006
--- work/xfce4-mailwatch-plugin-1.0.1/panel-plugin/mailwatch-mailbox-pop3.c	Sat Jun 30 20:26:56 2007
***************
*** 143,163 ****
  pop3_recv(XfceMailwatchPOP3Mailbox *pmailbox, gchar *buf, gsize len)
  {
      GError *error = NULL;
!     gssize recvd;
      
!     recvd = xfce_mailwatch_net_recv(pmailbox->sockfd,
!                                     &pmailbox->security_info,
!                                     buf,
!                                     len,
!                                     &error);
      
!     if(recvd < 0) {
          xfce_mailwatch_log_message(pmailbox->mailwatch,
                                     XFCE_MAILWATCH_MAILBOX(pmailbox),
                                     XFCE_MAILWATCH_LOG_ERROR,
                                     error->message);
          g_error_free(error);
!     }
      
      return recvd;
  }
--- 143,168 ----
  pop3_recv(XfceMailwatchPOP3Mailbox *pmailbox, gchar *buf, gsize len)
  {
      GError *error = NULL;
!     gssize recvd = 0;
!     gssize recvd_now;
      
!     do {
!       recvd_now = xfce_mailwatch_net_recv(pmailbox->sockfd,
! 					  &pmailbox->security_info,
! 					  buf + recvd,
! 					  len - recvd,
! 					  &error);
      
!       if(recvd_now < 0) {
          xfce_mailwatch_log_message(pmailbox->mailwatch,
                                     XFCE_MAILWATCH_MAILBOX(pmailbox),
                                     XFCE_MAILWATCH_LOG_ERROR,
                                     error->message);
          g_error_free(error);
! 	return recvd_now;
!       }
!       recvd += recvd_now;
!     } while (recvd <= 2 || buf[recvd-1] != '\n' || buf[recvd-2] != '\r');
      
      return recvd;
  }

Please use 'diff -u'; I can't really tell what changed there, and attach the patch using the link above rather than pasting in the comments box.

Created attachment 1274 
pop3 patch

I have the same problem with poczta.o2.pl server. I have done some testing with tcpdump and it appears that the initial response of the server is "+OK" and after that in another pocket the server sends rest of the line "Ready poczta.o2.pl". The Mail Watcher does not wait for that and sends the "USER foo" line before and probably treats the "Ready poczta.o2.pl" as a server response.
If I understand correctly the patch by Alec Panovici makes the Mail Watcher read the server responses until /r/n is found so this will fix problem with poczta.o2.pl too.
For testing you may use pop3 server poczta.o2.pl, user: bimbombam, pass: 0test5

Network code refactoring and partial rewrite is done.  Problems like this should be gone.  Please reopen if not.  If you can test svn trunk, please do.