Use a reputable mailing service.
If you’re hosting your own SMTP then you’re falling into the usual associated pitfalls. Mail delivery companies like mailgun and sendgrid use a number of techniques to ensure their servers are trusted and seen as reputable by destination networks. That’s why the documentation recommends you use one of those products over self-hosting.
Exactly - while you only send those messages in spikes, reputable third party mailing services can reliably deliver hundreds of thousands of emails into third party mailboxes every hour.
Unfortunately we can’t offer any assistance if you insist on running your own mail server. If you decide not follow the recommendations, that you accept the additional complexity it creates.