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

I need multiple IP addresses in different locations to provide redundancy against network outages

Why this is right
If all your traffic goes out via a single ISP and your connection to that ISP is eaten by a backhoe you’re  not going to be sending any email until that’s fixed. And from personal experience I know that can easily take two or three days for even minor fiber damage.
But be careful
You need to be sending email fairly consistently from an IP address in order to maintain a decent reputation for that mail source. If you treat a second location as a cold standby, only used when your main ISP breaks, expect to see serious delivery problems as you migrate across to it. Better to spread load across both locations, to keep both sets of addresses “warm” – but remember that that will halve the amount of traffic that a receiving ISP will see from any given IP address, which will change your decisions about whether to assign customers to a pool or not.
A better architecture
If all your production machines – smarthosts, webservers, databases – are hosted in a high quality datacenter run by an ISP with redundant connections to the Internet then you don’t need to worry about the redundancy yourself. If one of those connections is broken the ISP will route the traffic over a different connection to the same IP addresses mostly transparently. You won’t need to reconfigure anything, it’ll just keep working.
(Why don’t you just have redundant connections to servers hosted at your offices? First, it’s very expensive and time consuming to handle the mechanical aspects of ensuring that your two connections are really redundant, rather than being multiplexed onto the same fiber or running in the same conduit. Second, the smallest block of addresses you can multi-home in this way is 1000, and you can’t acquire those unless you’re already using more than 500 IP addresses efficiently.)
The weak point then is the connection between your offices and your datacenter that you need to administer the servers and provide customer support. But the IP addresses used for that don’t matter, so it’s easy and cheap to have a backup connection – even a cheap consumer cable or DSL connection. Or, if you’re a very small company, have your customer support folks use laptops and know which local coffeeshops and bars have free wifi.

backhoe • ESPs • godzilla • ip addresses