[CentOS] How to size an email server to handle 5 million emails per day

I have no idea as to how to size an email server. I was approached by
a customer that wanted a single server with RAID 1 disks to handle
about 5 million emails a day.
In general terms, what parameters should I take into account to size
the hardware specs when the average email is about 10kb, the smalles
email is 2kb and the largest email is about 5meg (with attachment)

thanks,

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Erick Perez