Why email deliverability has gotten harder, and what it means for retention

Major mailbox providers have progressively tightened authentication and engagement requirements for bulk senders, meaning email programmes that haven't kept sender reputation and list hygiene current are increasingly likely to land in spam regardless of content quality.

Sender authentication (SPF, DKIM, DMARC) is now closer to a baseline requirement than a nice-to-have, and a poorly configured setup can suppress deliverability across an entire retention programme without any obvious symptom beyond declining open rates.

List hygiene — actively removing or re-engaging unengaged subscribers rather than mailing the full list indefinitely — has become a deliverability requirement, not just a best practice, because engagement rate increasingly factors into whether mailbox providers trust a sender at all.