List of plugins used in project:

multiple-scms
mercurial
subversion
nant
sonar
flexible-publish
email-ext

I perform a fairly simple build, use the flexible-plugin to only archive on success and then send an email. Without "attach build log" checked everything works, with it checked I get the following error:

Run condition [Current build status] enabling perform for step [Archive the artifacts]
Archiving artifacts
Checking for post-build
Performing post-build step
Checking if email needs to be generated
Email was triggered for: Success
Sending email for trigger: Success
NOT overriding default server settings, using Mailer to create session
messageContentType = text/plain; charset=UTF-8
Request made to attach build log
Adding recipients from recipient list
Successfully created MimeMessage
Sending email to: MY.USERNAME@MYPLACEOFEMPLOYMENT.com
ERROR: Could not send email as a part of the post-build publishers.
java.lang.NullPointerException
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:311)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:289)
at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:249)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:718)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:693)
at hudson.model.Build$BuildExecution.cleanUp(Build.java:192)
at hudson.model.Run.execute(Run.java:1546)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:237)
Some error occured trying to send the email...check the Jenkins log