I worked for a company whose custom email campaign system took about a week to send out a campaign. This was a decade ago, so processing power has improved, but

I worked for a company whose custom email campaign system took about a week to send out a campaign. This was a decade ago, so processing power has improved, but I'm sure the mailing lists have grown as well. First, it took days to go through millions of data warehoused customer records and match them to cohorts, and then A/B variants within each cohort. Then, someone needed to spot-check the output to make sure it was good before the rack of mail sending servers could spend the next couple days sending them out. On a good day, I think each server managed to get out 10 emails a minute. Our IT department was adamant that there be an air gap between these public-facing "spam servers" and the data warehouse (our bread & butter), so a last-minute query to check for unsubscribes was out of the question. If you were assigned to a cohort on Monday and unsubscribed on Tuesday, you were still going to get that email Friday.