The conventional wisdom is to separate transactional and marketing onto different IPs. Transactional has high engagement, marketing has lower. Split them so marketing complaints do not pollute transactional reputation.
That advice is right at scale. At lower volumes, it is wrong.
Mailbox providers weight reputation by volume. An IP sending 200 transactional emails a day has barely enough signal to build any reputation at all. Splitting that traffic across two IPs leaves both with too-thin signal — neither can build the inbox placement you want.
Our rule of thumb: under 50k/day, share. Over 500k/day, split. Between the two, it depends on your complaint rate and your engagement.