Mimecast is my least favorite software our company has purchased. When installing it in your e-mail, it causes all links to first go through their server and then your webpage loads (causing a long stall). You have to register your various devices and browsers each time you use it, so if you are a private browser user, you must register every time. Even if you aren't, occasionally, it forgets your device so you must go through the process again. This alone makes this software unusable for me.
The attachment scanning portion is ok. I'm more concerned that folks aren't able to tell if something is a phishing scam; while they have gotten more sophisticated, perhaps training your company instead of installing prohibiting software is the way to go. It also doesn't actually catch the scams, it simply scans your files, so not really preventing your employees from clicking on anything malicious. (And again, this should be prevented with training and common sense).
I also dislike that they "hold" e-mails for you. Each morning, you get a digest that gives you your held e-mails; when the software was first implemented, it was holding e-mails that were important! Eventually, as you release and permit different senders, it won't hold them anymore, but you do have to tell it to do this. There may be a portal, but I don't have access to this so am unaware.