An SMTP server that requires lighting/bitcoin postage for every message it receives.
The service divides the postage with the recipient.
No spam filtering is done, leading to efficiencies.
The difficult part is not technical but social/economic: the network effect of non-postage SMTP is vast. By offering postage to any who receive the emails sent, participation could be incentivized. But adding a dependency on LN for the operation of the email system is a big/impossible ask. It would have to demonstrate substantial benefits over the status quo.