I have configured an external mail server in the mailer-plugin and I am able to send emails in the configuration menu ("Send email to test configuration").

My email configuration is as follows:
SMTP server: smtp.strato.de
Use SSL: yes
SMTP-Port: 465

But when a job fails and I have configured email notifications for this job, I do get the following error. Unfortunately, I haven't found any further logs related to this error.

Sending e-mails to: someone@gmail.com
ERROR: 530 5.7.0 User not authenticated

com.sun.mail.smtp.SMTPSendFailedException: 530 5.7.0 User not authenticated

at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2057)
at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTransport.java:1580)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1097)
at javax.mail.Transport.send0(Transport.java:195)
at javax.mail.Transport.send(Transport.java:124)
at hudson.tasks.MailSender.run(MailSender.java:130)
at hudson.tasks.Mailer.perform(Mailer.java:175)
at hudson.tasks.Mailer.perform(Mailer.java:138)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:741)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:690)
at hudson.model.Build$BuildExecution.post2(Build.java:186)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:635)
at hudson.model.Run.execute(Run.java:1843)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:429)
Finished: FAILURE

I have had this issue also with mailer-plugin 1.26 and 1.27.