The problem

In /com/atlassian/mail/server/AbstractMailServer.class the mail protocol is set to require authentication if a username and password is given:

if (StringUtils.isNotBlank(getUsername()))
{
  props.put("mail."+protocol+".auth", "true");
  isAuthenticating = true;
}

This is never set to false if the username and password are removed.

The solution

Ideally, change the code to this:

if (StringUtils.isNotBlank(getUsername()))
{
  props.put("mail."+protocol+".auth", "true");
  isAuthenticating = true;
}
else
{
  props.put("mail."+protocol+".auth", "false");
}

Workaround

1. Remove the mail server configuration from Admin > Mail Servers
2. Create a new configuration, ensuring that no username or password (including whitespace) is saved at all.

Steps to Reproduce

1. Have an SMTP mail server that accepts anonymous connections
2. Create a mail server in Confluence with a username and password, and save it
3. Remove the username and password and save
4. Expected behaviour: The authentication works anonymously
5. Actual behavior: The connection doesn't work and this stack trace occurs:

   2013-04-02 14:27:00,119 ERROR [scheduler_Worker-2] [atlassian.core.task.AbstractErrorQueuedTaskQueue] handleException com.atlassian.mail.MailException: javax.mail.AuthenticationFailedException: failed to connect, no user name specified?
   com.atlassian.mail.MailException: javax.mail.AuthenticationFailedException: failed to connect, no user name specified?
   	at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:193)
   	at com.atlassian.confluence.jmx.JmxSMTPMailServer.send(JmxSMTPMailServer.java:80)
   	at com.atlassian.confluence.mail.template.AbstractMailNotificationQueueItem.send(AbstractMailNotificationQueueItem.java:135)
   	at com.atlassian.confluence.mail.template.PreRenderedMailNotificationQueueItem.send(PreRenderedMailNotificationQueueItem.java:140)
   	at com.atlassian.confluence.mail.template.AbstractMailNotificationQueueItem.execute(AbstractMailNotificationQueueItem.java:106)
   	at com.atlassian.core.task.AbstractErrorQueuedTaskQueue$TaskDecorator.execute(AbstractErrorQueuedTaskQueue.java:107)
   	at com.atlassian.core.task.AbstractTaskQueue.flush(AbstractTaskQueue.java:45)
   	at com.atlassian.core.task.AbstractErrorQueuedTaskQueue.flush(AbstractErrorQueuedTaskQueue.java:37)
   	at com.atlassian.quartz.jobs.TaskQueueFlushJob.doExecute(TaskQueueFlushJob.java:32)
   	at com.atlassian.quartz.jobs.AbstractJob.executeInternal(AbstractJob.java:88)
   	at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:86)
   	at com.atlassian.confluence.setup.quartz.DelegatingClusterAwareQuartzJobBean.executeJob(DelegatingClusterAwareQuartzJobBean.java:16)
   	at com.atlassian.confluence.setup.quartz.AbstractClusterAwareQuartzJobBean.surroundJobExecutionWithLogging(AbstractClusterAwareQuartzJobBean.java:64)
   	at com.atlassian.confluence.setup.quartz.AbstractClusterAwareQuartzJobBean.executeInternal(AbstractClusterAwareQuartzJobBean.java:46)
   	at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:86)
   	at org.quartz.core.JobRunShell.run(JobRunShell.java:199)
   	at com.atlassian.confluence.schedule.quartz.ConfluenceQuartzThreadPool$1.run(ConfluenceQuartzThreadPool.java:20)
   	at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:549)
   Caused by: javax.mail.AuthenticationFailedException: failed to connect, no user name specified?
   	at javax.mail.Service.connect(Service.java:326)
   	at javax.mail.Service.connect(Service.java:176)
   	at javax.mail.Service.connect(Service.java:125)
   	at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:164)
   	... 17 more