Clients often struggle with spam placement despite gradual sending; DMARC, DKIM, and security practices remain crucial for deliverability.

You consult with them. You explain deliverability. You mention the importance of gradually increasing the sending volume. And then you see this.
And one week later:
aLex oUR EmAILS aRe gOiNG tO spAm AgaIn PleEz HelP