This information is from the Lexington Internet mailing list -- RCN is now blocking outgoing email (port 25, SMTP) so that you have to send email via their relay, smtp.rcn.com. See this announcement for more details. If you are currently setup to send email via your work mail server for example, you need to have a different setting for home. Or, just use SSH port forward to the same machine, which is what I do. I have a shell account at ISP X. Get an SSH client, either a commercial one like SecureCRT, or Putty freeware, and port forward port 25 to the mail relay (e.g. mail.x.com) at your ISP. Then change your email client to send out going email to "localhost".