This review was actually performed on Dec 3rd of 2018. It is located on this blog as a result of me migrating from Wordpress to Ghost.
This review is for their CloudFilter solution which is a cloud-based email security solution that we've had the pleasure of implementing for various customers.
We've mainly recommended and implemented MailProtector to help prevent spam and malicious emails in general from ever reaching any end users. One of the clients we implemented this for was receiving upwards of 100 different spam and malicious emails a day. Once they were brought on over to mailprotector they have yet to receive a malicious email. (Update 10/26/2019: they have received some, but nowhere near what they used to get daily)
MailProtector provides the end users with an informational email letting then know about any emails that were blocked/quarantined and provides them with the ability to release any messages that they do not believe are spam. This, of course, is an option that can be adjusted in the settings by the Administrator. For example, we do not allow any customers to release emails that were classified as being phishing attempts or that could contain malicious attachments. So far we've yet to see a malicious email successfully get through (not), however, we have seen some legitimate mailing lists get caught in the filter but it's usually a one time ordeal as future emails are allowed through after the user releases the first one from the quarantine.
MailProtector also allows for the viewing of stats as well as configurations on a domain by domain basis as well as a per user basis. This makes it super easy to find the most often targeted users and makes finding emails a breeze. The ease of implementation of the MailProtector solution is another big plus as it's a surprisinsgly seamless change and emails start flowing through without any issues.
Overall I'd have to say that MailProtector CloudFilter is definitely worth the cost, especially for organizations and users that constantly receive a lot of spam as well as malicious emails.
Update 10/26/2019: In reality, any mail filtering solution worth it's salt will do the same, if not better, and sometimes at a better cost. Find what works for you.