The ease of use, quality customer service from our account manager (Cheryl), ability to configure it for all my email addresses in one place at once! It's easy enough that any IT person can do this without needing training or support resources - which is great if you don't have those available yet :) I've been able get up-to date info about archival status across every single mailbox we send mail into since 2016 using their product alone!! There are no other products out there like theirs where they will archive your emails as soon as its sent so when someone needs them later down line they're still archived correctly even after being deleted/archived by another provider elsewhere before sending over here.
They also provide excellent technical assistance through twitter @entrustmailsupport who has always gotten back quickly whenever needed help solving problems via DM etc.