By default, Mimecast operates a strict greylisting policy for incoming email as well as scanning for SPAM and measuring reputation. What this means in practice is that Mimecast will deliberately delay high volumes of incoming email, sometimes for an hour or more and otherwise it might decide that emails that you are sending to your users are SPAM even if you know they aren't.
Individual Mimecast accounts can whitelist certain senders to bypass these checks but this will only apply to recipients within that account (not everyone using Mimecast) so if your scenario is to send lots of email to your own users within a Mimecast account, you are recommended to whitelist the SmartSurvey email servers for prompt delivery.
https://community.mimecast.com/s/article/Configuring-Permitted-Senders-Policies-1067720131
There are a number of options which you can use, although the easiest would be to specifically whitelist where the "From" address is the sending email you are using on your smartsurvey account e.g. surveys@example.com, that way you will not need to update the policy if our email servers change, which does happen. If you are unsure what the from address is, you can look at a previous message sent from SmartSurvey since the exact address will depend on whether you have sending domains/emails setup or not.
All of our servers are named e.g. ms1.ssmx.net, ms7, ms8 etc. but Mimecast doesn't currently support whitelist ing based on the sending server which means if you wanted to whitelist based on IP addresses, you would not only need to specify all of our email servers but it would also mean that if we added a new server, which we do from time-to-time, the policy would need to be updated or that new server would potentially suffer the same greylisting from mimecast.