How to Create Trigger Insert with a SELECT LEFT OUTER JOIN
Every trigger have some internal tables , INSERTED and DELETED.
In this case, we use only the INSERTED table, which hold the records inserted.

ALTER TRIGGER [xxx] ON [dbo].[Main] 
AFTER INSERT 
AS 
BEGIN

    SET NOCOUNT ON;

    INSERT INTO dtsx.dbo.main(refno,subject,recdate,dateind,officefrom,author,status)

    SELECT refno,subject,datereceived,'REC','TEST',author,StatDesc 
    FROM INSERTED AS I 
    LEFT OUTER JOIN dbo.DocStat AS DS
    ON I.Status = DS.StatID

END