By default, sendmail is disabled on Mac OS X 10.5 (Leopard). If you just want to enable/disable sending outgoing mail, run the following highlighted commands.

% sudo postfix start
Password:
postfix/postfix-script: starting the Postfix mail system

% netstat -an | grep LIST | grep 25
tcp4       0      0  127.0.0.1.25           *.*                    LISTEN

% sudo postfix stop
postfix/postfix-script: stopping the Postfix mail system

% netstat -an | grep LIST | grep 25
%

One of several possible error messages [that might have lead you to this page] is pasted below. The above solution should fix it.

javax.mail.MessagingException: Could not connect to SMTP host: localhost, port: 25;
  nested exception is:
 java.net.ConnectException: Connection refused
 at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1391)
 at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:412)
 at javax.mail.Service.connect(Service.java:288)
 at javax.mail.Service.connect(Service.java:169)
 at javax.mail.Service.connect(Service.java:118)
 at javax.mail.Transport.send0(Transport.java:188)
 at javax.mail.Transport.send(Transport.java:118)
        ...
        ...
Caused by: java.net.ConnectException: Connection refused
 at java.net.PlainSocketImpl.socketConnect(Native Method)
 at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
 at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
 at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
 at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:432)
 at java.net.Socket.connect(Socket.java:520)
 at java.net.Socket.connect(Socket.java:470)
 at com.sun.mail.util.SocketFetcher.createSocket(SocketFetcher.java:233)
 at com.sun.mail.util.SocketFetcher.getSocket(SocketFetcher.java:189)
 at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1359)
 ... 9 more