[eluser]richthegeek[/eluser]
A simple SQL (MySQL 5+, due to subquery) statement splits the list into groups of 5:
Code:
SELECT GROUP_CONCAT(email,',') AS recipients FROM emails GROUP BY MOD(id,(SELECT ROUND(COUNT(*) / 6) FROM emails));
You can add a WHERE statement between the FROM and GROUP, or a HAVING afterwards (remembering to add the appropriate column in the fieldlist) to limit by unsent.
Hope that helps.