No, excommunication does not put one outside the Church. Canon Law very clearly defines excommunication. It disallows the offender from receiving or administering the sacraments, and if the excommunication is ferendae sententiae, renders one from validly exercising governing authority in the Church.First, a valid excommunication puts one outside the Church.
Second, there is no salvation outside the Church.
Therefore, anyone under excommunication is in a very serious situation in regards to their salvation.
But yes, it puts the offender in a very serious situation, because an excommunication is always attached to a mortal sin, which can be ordinarily forgiven in Confession. Unfortunately, one cannot be absolved while excommunicated.