Adding a Mimecast email server into the instance might work but it might not sent the email to external email addresses outside the corporate emails
You will know you have to setup the ServiceNow umbrella account with Mimecast if
- You have setup an instance email account that uses an SMTP server with Mimecast
- Emails sent to external recipients are not receiving the messages
- The logs messages shows: 451 Open relay not allowed on some messages
Mimecast noted that multiple customers cannot use the same IP addresses. They need to setup the ‘authorized outbounds’. They use this umbrella account for ServiceNow and allow certain customers of ServiceNow to use the IPs included in the list.
Customers need to contact Mimecast support to request the use and provide them with the instance IPs in ServiceNow, so Mimecast can update the "umbrella account" themselves.
|Note: Please note Mimecast is not a ServiceNow product. The umbrella account is not owned by ServiceNow. For further details please contact Mimecast support|