Seharusnya seperti ini, Anda perlu memiliki kueri terpisah untuk mendapatkan tanggal maksimum (atau tanggal terakhir) email dikirim.
SELECT a.*, b.*
FROM tbl_customers a
INNER JOIN tbl_emails_sent b
ON a.customerid = b.customerid
INNER JOIN
(
SELECT customerid, MAX(datesent) maxSent
FROM tbl_emails_sent
GROUP BY customerid
) c ON c.customerid = b.customerid AND
c.maxSent = b.datesent