Google (InboxAutomate)
$15 per domain per year + $4 per inbox per month (no minimum order required).
Outlook (InfraInbox)
Users can purchase a minimum of 2 domains, up to 20 domains. You can create 3-5 mailboxes per domain. Combining above, an user can create select anywhere between 6 mailboxes to 100 mailboxes.
It will cost $16 per domain per year + $5 per month per inbox.
SMTP (Mailreef)
$19 per domain per year + $3.99 per mailbox per month, with a minimum order of 15 domains and 5 mailboxes per domain (total minimum of 75 mailboxes).