When a Case has been created via Email-to-Case and the customer responds to the Case by an email, that email might trigger multiple Case email notifications to Case Owner or Queue.
The notification emails received will look something like below:
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From: Case Notification <noreply@salesforce.com>
Sent: Thursday, November 29, 2018 12:33 PM
To: Billing <billing@abcxyz.com>
Subject: New case email notification. Case number xxxxxx
An email has been received for case xxxxxx: RE: Email Subject.
Please click the link below to review the new email and respond appropriately.
https://naxx.salesforce.com/02sxxxxxx
NOTE: It is not recommended that you forward this email because its association with the original case may be lost.
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This article explains why an email reply to an Email-to-Case case can trigger multiple duplicate notification emails, and how to permanently prevent this email loop.
This occurs when 'Notify Case Owners on New Emails' is enabled in the Email-to-Case settings, and the Email-to-Case routing address (the support mailbox address) is also the Case Owner/Queue email address, or matches the Default Case Owner user's email address.
Because 'Notify Case Owners on New Emails' is enabled, Case owners automatically receive a notification whenever a new email arrives on their case. If the Default Case Owner's or Case Owner/Queue's email address is the same as the Email-to-Case routing address, that notification email is itself sent to the routing address. Salesforce then treats the notification as a new customer reply and routes it back into the case, creating an email loop that triggers repeated, duplicate notifications.
Always configure a unique routing email address for Email-to-Case services. This address should never also be used as a Case Queue email address or assigned to any Salesforce user, including the Default Case Owner.
For more detail on configuring these settings, see Email-to-Case Settings.
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