"1TB of storage space because emails are very important to them."
Is this 1,000 users with 1Gb Mailboxes, all full, or 10 staff with 100Gb mailboxes each?
Perhaps ALL the emails have mid-size word docs, so each email is 500Mb. That's storage for 200,000 emails. Will the two hundred thousand word documents be read by an automatic system?
Just an observation, but if your client really does legitimately need a terabyte of storage for mail, it's pretty serious business, if that's the case then you should install more than one mail server for redundancy.