A user in crowd can have no e-mail-address configured.
If a user without an emailaddress is included in the notification scheme, the message jira wants to send ends up in the error queue. The following error comes from the jira logfiles.

The problem is, that the e-mail is sent to all the other users in the notification scheme. They end up being e-mailbombed because jira did not stop trying to mail this message.

A shutdown/restart of tomcat emptied the mail queue of jira for now.

2007-09-10 15:19:08,494 Sending mailitem com.atlassian.jira.mail.IssueMailQueueItem@ca8ad4[issue=com.atlassian.jira.issue.IssueImpl@e72595[id=12566,summary=Instrument Object implementation,key=AAA-7,created=2007-09-06 15:46:45.415,updated=2007-09-10 14:56:48.001,assignee=pietje,reporter=puk],remoteUser=pietje,notificationType=Project_Role,eventTypeId=2,templateId=2] ERROR [atlassian.jira.mail.IssueMailQueueItem] 'To' is a required field
java.lang.IllegalArgumentException: 'To' is a required field
at com.atlassian.mail.Email.<init>(Email.java:51)
at com.atlassian.jira.mail.Email.<init>(Email.java:18)
at com.atlassian.jira.mail.MailingListCompiler.createEmail(MailingListCompiler.java:284)
at com.atlassian.jira.mail.MailingListCompiler.addMailsToQueue(MailingListCompiler.java:236)
at com.atlassian.jira.mail.MailingListCompiler.access$200(MailingListCompiler.java:35)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.addEmailsToQueue(MailingListCompiler.java:405)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendLists(MailingListCompiler.java:378)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendNoLevelsIgnoreGroup(MailingListCompiler.java:359)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendToAll(MailingListCompiler.java:354)
at com.atlassian.jira.mail.MailingListCompiler.sendLists(MailingListCompiler.java:118)
at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:167)
at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:68)
at com.atlassian.jira.web.action.admin.MailQueueAdmin.doExecute(MailQueueAdmin.java:38)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)
at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:54)
at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:132)
at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:209)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.AccessLogFilter.doFilter(AccessLogFilter.java:73)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:119)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:55)
at com.atlassian.jira.web.filters.SitemeshExcludePathFilter.doFilter(SitemeshExcludePathFilter.java:38)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:182)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:181)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132)
at com.atlassian.jira.web.filters.JIRAProfilingFilter.doFilter(JIRAProfilingFilter.java:16)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.ActionCleanupDelayFilter.doFilter(ActionCleanupDelayFilter.java:43)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.RequestCleanupFilter.doFilter(RequestCleanupFilter.java:49)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:91)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.gzip.GzipFilter.doFilter(GzipFilter.java:72)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.appconsistency.db.DatabaseCompatibilityEnforcerFilter.doFilter(DatabaseCompatibilityEnforcerFilter.java:39)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
2007-09-10 15:19:08,495 Sending mailitem com.atlassian.jira.mail.IssueMailQueueItem@ca8ad4[issue=com.atlassian.jira.issue.IssueImpl@e72595[id=12566,summary=Instrument Object implementation,key=AAA-7,created=2007-09-06 15:46:45.415,updated=2007-09-10 14:56:48.001,assignee=pietje,reporter=puk],remoteUser=pietje,notificationType=Project_Role,eventTypeId=2,templateId=2] ERROR [atlassian.mail.queue.MailQueueImpl] Error occurred in sending e-mail: com.atlassian.jira.mail.IssueMailQueueItem@ca8ad4[issue=com.atlassian.jira.issue.IssueImpl@e72595[id=12566,summary=Instrument Object implementation,key=AAA-7,created=2007-09-06 15:46:45.415,updated=2007-09-10 14:56:48.001,assignee=pietje,reporter=puk],remoteUser=pietje,notificationType=Project_Role,eventTypeId=2,templateId=2]
com.atlassian.mail.MailException: 'To' is a required field
at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:178)
at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:68)
at com.atlassian.jira.web.action.admin.MailQueueAdmin.doExecute(MailQueueAdmin.java:38)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)
at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:54)
at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:132)
at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:209)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.AccessLogFilter.doFilter(AccessLogFilter.java:73)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:119)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:55)
at com.atlassian.jira.web.filters.SitemeshExcludePathFilter.doFilter(SitemeshExcludePathFilter.java:38)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:182)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:181)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132)
at com.atlassian.jira.web.filters.JIRAProfilingFilter.doFilter(JIRAProfilingFilter.java:16)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.ActionCleanupDelayFilter.doFilter(ActionCleanupDelayFilter.java:43)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.RequestCleanupFilter.doFilter(RequestCleanupFilter.java:49)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:91)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.web.filters.gzip.GzipFilter.doFilter(GzipFilter.java:72)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:37)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at com.atlassian.jira.appconsistency.db.DatabaseCompatibilityEnforcerFilter.doFilter(DatabaseCompatibilityEnforcerFilter.java:39)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
Caused by: java.lang.IllegalArgumentException: 'To' is a required field
at com.atlassian.mail.Email.<init>(Email.java:51)
at com.atlassian.jira.mail.Email.<init>(Email.java:18)
at com.atlassian.jira.mail.MailingListCompiler.createEmail(MailingListCompiler.java:284)
at com.atlassian.jira.mail.MailingListCompiler.addMailsToQueue(MailingListCompiler.java:236)
at com.atlassian.jira.mail.MailingListCompiler.access$200(MailingListCompiler.java:35)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.addEmailsToQueue(MailingListCompiler.java:405)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendLists(MailingListCompiler.java:378)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendNoLevelsIgnoreGroup(MailingListCompiler.java:359)
at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendToAll(MailingListCompiler.java:354)
at com.atlassian.jira.mail.MailingListCompiler.sendLists(MailingListCompiler.java:118)
at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:167)
... 57 more