It is possible for the JIRA mail queue to get stuck if:

• in a notification scheme, a group is notified of an event
• the group is deleted

JIRA then falls over trying to notify a nonexistent group:

java.lang.NullPointerException
at com.atlassian.core.user.UserUtils.getUsers(UserUtils.java:118)
at com.atlassian.jira.notification.type.GroupDropdown.getRecipients(GroupDropdown.java:36)
at
com.atlassian.jira.notification.DefaultNotificationSchemeManager.getRecipients(DefaultNotificationSchemeManager.java:94)
at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:100)
at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:55)
at com.atlassian.jira.service.services.mail.MailQueueService.run(MailQueueService.java:25)

This bug is fixed in 2.5.2 (at least - probably earlier), but apparently still suffers from the symptom if old data is imported.