Corporate email admins can be more aggressive and unpredictable than big ISPs or Google or Microsoft, because they have a more direct relationship with their fellow staff. They can train users how to manage their own whitelists, and they usually already have an IT helpdesk staffed answer questions and concerns from users.

Corporate email admins can also be opinionated about email senders because their email system is only for their employees' work, specifically. They are less likely than Gmail to care about email from the little guys who are trying to build an audience or business. They definitely don't see it as their job to provide startups and random people with access to the inboxes of their staff. They will err on the side of saying no to unfamiliar email.

Corporate email is where it gets hard. You can have a clean IP, all the authentication options configured correctly, and still struggle to get out of the spam folder. This is one reason businesses will pay for big-name email platforms, which have spent a lot of time and money building good reputations with corporate teams, and have people who specialize in maintaining those reputations.