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Debug a JavaMail ProgramTag(s): Networking

JavaMail Debug mode
To set the JavaMail Debug mode "on" :
Session mailSession = Session.getDefaultInstance(props, null);
or set the property when launching the JVM
java -Dmail.debug=true ...
This setting puts the JavaMail classes in debug mode mode to System.out.

To redirect the JavaMail debugging output to a more appropriate log file you can

  • link a PrintStream to a ByteArrayOutputStream,
  • tell to JavaMail to use your PrintStream,
  • do the JavaMail stuff,
  • dump the the content of the ByteArrayOutputStream to your favorite logger.
   ByteArrayOutputStream os = new ByteArrayOutputStream();
   PrintStream ps = new PrintStream(os);
   Session mailSession = Session.getDefaultInstance(props, null);
   try {
     if (MAIL_DEBUG) {"JAVAMAIL debug mode is ON");
     if (MAIL_DEBUG) {; }
   finally {
Verify connectivity to the MailServer with Telnet :
telnet mymailserver 25
for example, you can detect if your firewall is blocking your connection.

Windows 7
By default, the telnet client is not installed on a Win7 workstation. To installed it, open command shell and type :

pkgmgr /iu:"TelnetClient"
You can also install it through the Control Panel, see Microsoft Technet.
Use a JavaMail server mock-up to act as "in-memory" mail server
See and These mock-ups are designed to act a mail server but the actual email is not delivered to the mail recipient which can be useful in a testing stage.