Sql Server Join

Hi Experts
I want to do left join and Join based on Case.
Basically requirement is I want to see the result All Record ,mail sent to airtel and not sent record,
if mail sent then IRActivatedPlanDetails table must have record.
not sent IRActivatedPlanDetails not have record.
Declare @MailSentToAirtel bit=1;
SELECT *
FROM IR.TravelRequestMst MST     (CASE @MailSentToAirtel     WHEN @MailSentToAirtel IS NULL OR @MailSentToAirtel=0     THEN     LEFT JOIN IR.IRActivatedPlanDetails IRTR ON MST.TravelRequestMstID = IRTR.TravelRequestMstId     ELSE     INNER JOIN IR.IRActivatedPlanDetails IRTR ON MST.TravelRequestMstID = IRTR.TravelRequestMstId     END     )