Almost everything written about email talks about the content of the message. That is the last problem. The first is technical: a domain with no SPF, no DKIM and no DMARC has its messages filed as spam whatever the text says, and nobody warns you. So we start by setting those three records and checking they line up, which takes a morning and changes everything.
Then comes reputation, which is built and destroyed quickly. Sending to ten thousand addresses at once from a new domain is the surest way to be flagged: so we raise the volume gradually, over three to six weeks, watching the complaint rate. Above three complaints per thousand a provider starts filtering; above five it blocks. Those thresholds are not negotiable and no amount of creativity compensates for them.
Content comes only after that, and it follows one rule: one message, one action. A letter offering six things gets none of them done. So we write short messages with a single button, and we measure clicks rather than opens, which have become unreliable since Apple pre-opens its users' mail. A flattering open rate no longer means anything; a click rate is still a fact.