I need IP addresses to handle the volume

Number two of seven in our occasional series on why ESPs need, or don’t need, lots of IP addresses to send mail properly.

I need lots of IP addresses so my MTAs can handle the volume of mail sent

Why this is right
One IP address per outbound smarthost is a sensible minimum. It is possible to set up multiple smarthosts behind a single IP address using a proxy server or reverse load balancer, and some organizations do that, but it makes it much harder to diagnose some sorts of operational problems. If the different smarthosts behind the proxy use different hostnames then the externally visible behaviour will be a single IP address HELOing as many different machines – which is behavior that is otherwise distinctive to spam sent from botets of infected machines, so will lead to mail being blocked. If, instead, they all use the same hostname then it’ll be hard to say which one of the cluster of smarthosts sent a message if there is a problem that needs to be diagnosed later.
So if you need 50 smarthosts to handle the volume of email you’re sending out, that’s really good justification for needing 50 IP addresses.
Why this is wrong
A typical smarthost can send an awful lot of email if it’s configured correctly. I tried to find some hard numbers on that, but  smarthost vendors don’t seem to publish their numbers much, and most of the trustworthy benchmarks I found published were from some years ago, when servers were a tenth of the speed they are now.

Server	Message Size	Delivery rate
Postfix on old 300MHz x86	4k	450,000/hour
MS Exchange 2003 on quad 1.6GHz server	50k	510,000/hour
Sendmail Sentrion MP302	?	212,000/hour
Sendmail Sentrion MP301	?	120,000/hour

Do I trust all these numbers? Not necessarily, and they’re not directly comparable, but they all look quite plausible from a software engineers point of view. (It would be fascinating to get some performance numbers from vendors that are comparable, though. A smarthost-for-bulk-mail benchmark. Hmm….).
Edit: I’ve confirmed that Message Systems can exceed 1,500,000 deliveries an hour on a well provisioned server – and that’s while providing all the knobs and reports and monitoring that make managing delivery at any sort of volume much easier than a dumb smarthost.
But if you assume that you can send even 100,000 50k messages per hour from one smarthost, that’s more than enough mail to saturate a ten megabit connection, and to send seventy million emails / month – for each smarthost.
There are good reasons to have more smarthost capacity than you’re using, and to have more than one IP address per smarthost, but pure MTA capacity is hardly ever going to be justification for using more than a small handful of IP addresses to send mail from.