Case attachment, comment and assignment emails are no longer relayed after Spring '16 release
|Knowledge Article Number||000232713|
|Description||In the Spring '16 release adjustments were made to send emails that respect the setting "Send Case Notifications from System Address" from a System binding IP. Consequently, users may notice these emails are no longer sent to a configured "Email Relay server" or the sending IPs for the emails change from the ones users are used to receiving them from. "System" emails are sent from the domain salesforce.com and are sent to users based on the normal MX lookup functionality of a regular email from a domain.
To modify this behavior back to the prior functionality in Setup search for "Support Settings" and un check the box "Send Case Notifications from System Address"