I've been using Hudson 1.219 with the regular mail notification for a couple of
days now without problems.
Today, I installed the new mail-ext plugin (version 2.1). I haven't used that
plugin before.
Here's the plugin configuration:
Global Mail address => my mail address
For testing, I want to receive mails, when a build runs successfully, so I did
the following:

• I activated the build trigger "Success".
• I deactivated "Send to committers"
• I activated "Send to recipient list"
• I left the field for the recipient list empty, as I expected that the mail
  will always be sent to the global recipient list.
• Even when I fill the recipient list, the NPE still occurs.

Now the project fails due to a NPE in the plugin, here's the stack trace from
the end of the console log:

Email was triggered for: Success
There are 1 triggered emails.
Sending email for trigger: Success
FATAL: null
java.lang.NullPointerException
at
hudson.plugins.emailext.plugins.content.HudsonURLContent.getContent(HudsonURLContent.java:20)
at
hudson.plugins.emailext.ExtendedEmailPublisher.replaceTokensWithContent(ExtendedEmailPublisher.java:360)
at
hudson.plugins.emailext.ExtendedEmailPublisher.replaceTokensWithContent(ExtendedEmailPublisher.java:362)
at
hudson.plugins.emailext.ExtendedEmailPublisher.transformText(ExtendedEmailPublisher.java:340)
at
hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:295)
at
hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:265)
at
hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:257)
at
hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:215)
at
hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:309)
at
hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:297)
at hudson.model.Build$RunnerImpl.post2(Build.java:118)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:282)
at hudson.model.Run.run(Run.java:774)
at hudson.model.Build.run(Build.java:85)
at hudson.model.ResourceController.execute(ResourceController.java:70)
at hudson.model.Executor.run(Executor.java:82)