Problem

Email notifications work, but throw exceptions in the logs. The following appears in the atlassian-confluence.log:

2015-05-07 13:41:30,354 ERROR [NotificationSender:thread-2] [plugin.notifications.dispatcher.NotificationErrorRegistryImpl] addError Error sending notification to server '<Unknown>'(-1) for INDIVIDUAL task (resent 4 times): Error generating message for server 'System Mail' on medium 'mail' for user 'ff808181442f17b001442f198675309b'.
NoSuchElementException: 
 -- referer: https://localhost:8090/confluence/pages/editpage.action?pageId=360452 | url: /pages/doeditpage.action | userName: admin | action: doeditpage | page: 360452

Diagnosis

Environment

• Users are sourced from Active Directory (or possibly another LDAP system)

Diagnostic Steps

• Check the status of the user mentioned in the error. Is the user currently active or disabled? 

Cause

The error is thrown because the user, which is synchronized from Active Directory (or another external LDAP), is no longer active and has been disabled in LDAP, and by extension in Confluence. However, the user is still subscribed to page or space notifications, and being disabled does not remove those subscriptions. Confluence continues to try to send notifications to that user, and because the user is now disabled, it triggers a NoSuchElement exception.

Resolution

Remove the space/page watches that the disabled user is subscribed to. You can do this by querying the notifications table in the database:

• Confirm that the disabled user is subscribed to notifications. Using the user key from the error:

  SELECT * FROM notifications where username = 'ff808181442f17b001442f198675309b';
  

• Delete that user's notifications:

  DELETE FROM notifications WHERE username = 'ff808181442f17b001442f198675309b';
  

  Always back up your database before performing any modification to the database. If possible, try your modifications on a test server.

• Clear the Confluence caches at Confluence Administration > Cache Management, or restart Confluence.