Symptoms

1. One or more mail messages are not being sent.
2. Indexing has some problem completing, resulting in search inconsistencies and outdated Recently Updated results.

The following appears in the atlassian-confluence.log (or atlassian-confluence-outgoing-mail.log from 7.11)

serviceAction Could not execute action
 -- url: /admin/mail/resenderrorqueue.action | userName: xxx
java.lang.ClassCastException: com.atlassian.confluence.mail.template.ConfluenceMailQueueItem
	at com.atlassian.confluence.admin.actions.mail.ViewMailQueueAction.doResend(ViewMailQueueAction.java:54)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

Cause

An import of external mailboxes resulted in corrupt email addresses. Notifications are being sent to an invalid address and failing. Valid emails may not be sent until the message with the invalid email is sent.

Resolution

1. Track down any invalid email addresses by looking in The Mail Queue.
2. Check for the invalid addresses
3. Remove the associated queue entries
4. Flush the mail queue