Our basic email plan sends all email from a single IP address: the IP address of the server that your content is being hosted on. While we have anti-measures in place, the fact is that someone is highly likely to abuse this service in the future which will cause the shared IP address to become blacklisted by some providers; being blacklisted means that your email won't reach any mailboxes hosted by said provider. Our pro email on the other hand sends emails via an external service called MailChannels. MailChannels monitor their IP pool and ensure that none of their IPs are blacklisted on any public register. Unless you're sending spam, your email will definitely get delivered via this service (unless you're not on the recipient's whitelist). The two services will be combining in the near future.
The basic email plan is included with all hosting plans (in varying tiers), and the pro email is sold seperately by storage required rather than by quantity of user accounts.