All email notifications and scheduled reports suddenly stop being delivered.
Contacting Workfront support and they were able to look at the email logs and see this Mimecast Error.
Email 550 Administrative prohibition - envelope blocked - https://community.mimecast.com/docs/DOC-1369#550
One cause of this may be if your network email management software has blocked email coming from Workfront. This can happen automatically by your software attempting to protect your email system. For example: Mimecast will think because Workfront sends so many email notifications and scheduled reports that it must be spam. Mimecast will block all email from Workfront.
Check Mimecast's "Blocked Sender's Policies" make sure that Workfront is allowed to email and is not being blocked. See Mimecast document here: https://community.mimecast.com/docs/DOC-1388-blocked-senders-policies
In one specific instance of this problem the customer contacted Mimecast and they determined they needed to change smart hosts in their MX records. Please contact Mimecast for more details and specifics on this Error.