How do different ISPs respond to segmentation differences?

Not all mailbox providers weight the same signals the same way, and your segmentation strategy needs to account for that. Sending a highly engaged segment and a barely-engaged segment in the same campaign looks fine on average. But it can cause inbox placement problems at specific providers, especially the ones that apply domain-level engagement filters.

Gmail is the most sophisticated about engagement signals. It applies reputation at the domain level, watches click-to-open ratios closely, and uses machine learning to decide where each message lands. Primary inbox, promotions tab, or spam. Sending a low-engagement segment alongside a high-engagement one in the same campaign can drag down your overall engagement rate for that send, and Gmail may start routing more of your mail to the promotions tab or spam as a result. The fix is to either suppress low-engagement contacts or send them separately on a lighter cadence.

Outlook and its Postmaster tools focus more on authentication (SPF, DKIM, DMARC) and infrastructure reputation. They're somewhat less sensitive to individual engagement patterns than Gmail, but will throttle senders who accumulate high complaint rates from their user base. Yahoo Mail is complaint-driven. Being flagged as spam by Yahoo users has outsized effects on your reputation specifically with Yahoo and AOL, which share filtering infrastructure.

The practical guidance: segment your list into at least two tiers, engaged (opened or clicked in the last 60 to 90 days) and unengaged, and send them on separate schedules or with different warmup pacing for new sending infrastructure. Monitor inbox placement by provider using a seed-testing tool to see if segmentation changes move the needle at specific mailbox providers.