I can receive mail, but not send
Usually this happens to people who are checking their Case email using an outside Internet Service Provider (ISP). Most Internet Service Providers require you to use their server to send email. If this is the case for you, simply call your ISP to find out what their Send Mail Server is called and in your email program enter it in the settings where smtp.case.edu is currently.
Otherwise try deleting any queued (unsent) messages in your Out box. If there is a problem with one of our outgoing messages most email programs will prevent any others from sending as well. Possible problems with outgoing messages include the size of the message (There is a size limit on outgoing messages.) or a bad email address.