Given your example, 60 customers can send 1M emails each at the same time, case in which that server isn't enough.
They can send 1M emails in a day, case in which the server is enough.
You'll have to just go with it and see how it works.
P.S:
If all those customers will be very active, i'd suggest putting mysql on a separate box and give it as much ram as possible, of course, if this setup is possible for you.