Love it or loathe it, email isn't just a pile of messages or your badly disorganized to-do list.

"Email is about half of your corporate memory," claims Mimecast's co-founder and CTO Neil Murray (the rest of it is on your file servers tucked away in all the documents everyone in your company has created).

That's a good reason for moving to cloud email services like Office 365, because that way you're not faced with the temptation of throwing away what is valuable information just to save the cost of putting bigger hard drives in the mail server.

But if you really want to mine the hidden treasures of relationships, expertise and sentiment hidden in your message archives - or just do a better job of ediscovery than Exchange does alone - an archiving service like Mimecast (which works with Office 365 as well as on premise Exchange servers) is your next step.

Yes, you can look back at old messages in Exchange Online and use keywords to set up legal holds, but Exchange Online is a cloud service, not an archive. Want to go back and investigate an allegation? "Microsoft can't reconstitute a mailbox as it was two years ago," Murray points out. "You don't know if the data is the same as what was there yesterday." Plus the amount of control that email administrators have with Office 365 means they could permanently delete messages.

It might not sound like it, but Murray is a fan of the Microsoft cloud service. "We assume all our customers will move to Office 365," he says and although Mimecast can connect to Google email and will set that up if customers want it, he's unenthusiastic. "Google is not our focus. We prefer to work with the big guys [in enterprise email] and that's Microsoft."

It's a question of scale, for Mimecast and for the businesses they see choosing Office 365. "You don't want 10 vendors in the cloud," Murray says; "you might want one, Microsoft - but can they solve all your problems."

With Microsoft's expertise in both email and security, Exchange Online will be better run and more secure than the mail systems at most companies; unless you believe you can protect your IT infrastructure as well as a bank does, he predicts you'll be better off with Office 365 for mail. But switching to Office 365 does bring risks with it that you need to plan for - because using a single service, even a cloud service, is always a risk.

"I talk to businesses moving to the cloud and I ask them what their business continuity plan and their disaster recovery plan for Office 365 is. They usually assume it's not their problem, because Office 365 has an SLA." But cloud services do have outages, he points out, "and the SLA doesn't compensate you for business losses."

Not only does Mimecast keep an archive of your messages on a separate service, you can also use to send and receive messages if Exchange Online isn't available. "We can keep you emailing in an Office 365 outage."

For the majority of the time, when Office 365 is working, Mimecast will collect email in the background, as it arrives on your Exchange server, but it's still useful. Instead of searching in Outlook, you can explore your messages through your Mimecast 'virtual drive' of messages file by metadata (including through mobile apps). Want to see all the messages you've exchanged with people at a specific company in a six-month period? That's a simple query.

The next step is mining the big data hidden in those messages. Mimecast has an API that you can use through a service like Salesforce to reveal what you don't know about your customers and clients, based on the content of messages, the pattern of replies or even when someone stops replying. "Is someone senior about to resign? Is a customer of yours unhappy and not going to review?" You might not think to go check how many messages a valued customer has sent to your support team without getting an answer, but if you're expecting them to pay you for another year, you probably want to know if they're starting to sound frustrated about it.

To be able to get that kind of information, you need to be archiving all your company messages rather than deleting them to save space. The best way to get businesses to make that shift is to show them quantifiable benefit, but Murray admits "it's an uphill battle to move people from discarding to keeping messages".

He's also looking for new ways to protect email by understanding the contents. It would be particularly easy to block phishing messages that try to get your mail credentials because Mimecast knows whether a URL really goes to your Outlook Web Access page or if it just looks as if it does (because Mimecast connects to that same URL).

His latest idea is something he calls "the human firewall". Never mind making email security better; he's asking if we can we upgrade the human users? He envisages a tool that pops up when you click a link in an email that asks you "Where did you think this link was going?" as a way of raising awareness about possible threats. Because if you'll only be able to mine the contents of email if we don't all abandon it as too dangerous because of all the phishing messages.