select      customer
           ,count (*)   as contacts

from        mytable

where       contact_date >= current_date() - 90

group by    customer
;

MarketingEmails( CustomerID, Email, SentTo, SentDate );


SELECT CustomerId,
       Email,
       SentTo,
       SentDate,
       COUNT(1) OVER ( PARTITION BY CustomerID,Email ORDER BY SentDate ) AS
       CountofContactsbyCustomerEmail
  FROM MarketingEmails
 WHERE DATEDIFF(dd,GETDATE(),TO_DATE( SentDate ) ) >= 90;