I confirm that warming up is not just trendy term, but really necessary thing. I also used to think that it was only for new mailboxes, but it turned out that it helps old ones too. Especially if you haven't written for long time and then suddenly start sending. Without warming up, you would immediately end up in spam. But with a good service, like snov.io, everything goes smoothly and stably. The main thing is not to rush and let the mail restore its reputation.