Number three of seven in our occasional series on why ESPs need, or don’t need, lots of IP addresses to send mail properly.
I need many IP addresses so that I can work around ISP throttling limits
Why this is right: There are ISPs that limit the number of emails that can be sent from a particular IP address in a given time period to quite a low level, as low as 1000 emails per hour per IP address in some cases. If that’s a mid-sized ISP with perhaps a million users and an ESP customer is sending email to 5% of their user base (not unreasonable for some customers) then that would take more than two days to send a news letter to those recipients, assuming absolutely perfect management of send rate. With more realistic inefficiencies for send rate management it could easily be a week. Using multiple IP addresses to spread the traffic in that case seems perfectly reasonable, though it would make everyone involved happier if the ISPs used more reasonable rate limiting metrics, perhaps tied to sending IP address reputation rather than applied globally to all non-whitelisted senders.
Why this is wrong: Not every customer will be trying to send huge volumes of email to a receiving ISP that throttles inbound mail. For dedicated IP customers there’s no need to give them extra outbound IP addresses for this reason unless it actually becomes a real operational problem - if it takes an hour or two for a senders maildrop to reach the inbox at a particular ISP that’s not an operational problem. For pooled IP customers you may need to add IP addresses to deal with this, but only to the extent it’s needed to keep delivery times for pool customers to that recipient ISP reasonable. And two or three hours is not unreasonable.
Why else is this wrong: Naive throttling of this sort, combined with the obvious engineering changes needed for senders to work around it hurts everyone – senders, recipients, receiving ISPs. You might have to work around it in the short term, but in the longer term work with receiving ISPs to resolve the problem in a way that makes everyone happier, whether that be whitelisting your outbounds or moving to a reputation adaptive throttling approach. Of course, if they tell you that your mail is throttled because it has a poor reputation you need to fix that before doing anything else.