I realized this issue is actually with parameterized build trigger. It only is visible when the upstream job is a multi-configuration project.

So, this can be closed? Or should it be reassigned to the parameterized build trigger plugin?

I have two free-style projects. The upstream project has a manual promotion process that triggers the downstream project. Jenkins default promotion action Build other projects works as expected whereas promotion actions Trigger/call builds on other projects and Trigger parameterized build on other projects by Parameterized Trigger plugin result in the following exception in the downstream project when trying to send an email to requesters. There is no additional info in jenkins log file, it just repeats the stack dump.

Jenkins 1.480.3, Email-ext 2.28, Parameterized Trigger 2.17

Email was triggered for: Success
Sending email for trigger: Success
NOT overriding default server settings, using Mailer to create session
messageContentType = text/html; charset=UTF-8
Adding recipients from recipient list
Sending to requester
ERROR: Could not send email as a part of the post-build publishers.
java.lang.NullPointerException
	at hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:537)
	at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:304)
	at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:296)
	at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:256)
	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
Finished: SUCCESS

• Does it always happen?
• Would you attach build.xml of the build raised the error?

The problem used to be permanent in my case. The good news is that I cannot reproduce it anymore. I tried to reproduce it in two different environments without luck:
Env 1: Jenkins 1.509.4 + Email Extension 2.36 + Parameterized Trigger 2.22
Env 2: Jenkins 1.532.2 + Email Extension 2.37.2 + Parameterized Trigger 2.22

I cannot comment on the original problem regarding multi-configuration projects though.

I could not reproduce the problem, too.
Jenkins 1.532.1 + Email-ext 2.37.2 + Parameterized-trigger 2.22.

What I tried:

• Run parameterized trigger without parameters as a publisher from multi-configuration project, and always send email to requester in the downstream.
• Run non-blocking parameterized trigger without parameters as a builder from multi-configuration project, and always send email to requester in the downstream.
• Run blocking parameterized trigger without parameters as a builder from multi-configuration project, and always send email to requester in the downstream.

I think it is already resolved in some version:

• There is another ticket JENKINS-12300, which seems same to this and it is reported as resolved (I'm not sure how it is resolved).
• Current codes of email-ext seems completely different from that of Email-ext 2.28.

I'll close this ticket as Resolved if there is no new reports in one month.