It’s actually both, server IP and domain, separately, both have their own reputation and if either is deemed spammy, your mails will be sent to spam.
Source: Am an architect and a dev for a newsletter provider, we deal with reputation issues multiple times a month. Usually because one of our clients, who definitely never bought any database of contacts from a shady origin, hit a spam trap which immediately sends your reputation to red numbers.
It’s actually both, server IP and domain, separately, both have their own reputation and if either is deemed spammy, your mails will be sent to spam.
Source: Am an architect and a dev for a newsletter provider, we deal with reputation issues multiple times a month. Usually because one of our clients, who definitely never bought any database of contacts from a shady origin, hit a spam trap which immediately sends your reputation to red numbers.