I suppose you might have to explain what you mean by “valid”. I don’t think that there is an exact correlation of that term in Orthodoxy.
When Roman Catholics refer to other church’s sacraments as “invalid” or “not valid” what does that mean? Does it mean that you know it’s a fake…unreal?
Let us say, for instance, communion in a Methodist church. Or perhaps Anglican orders of the priesthood. When your church states that these are not valid, are you stating that without qualification that they are fake, or…imitation sacraments?
Because that is not the position of Holy Orthodoxy.
If we are not in communion, it is because your church and mine do not teach the same things and you and I do not believe the same things.
Therefore, from my perspective as Orthodox you are not in Holy Orthodoxy. You are not in the church which for all practical purposes I am concerned with. I cannot know anything about the orders and sacraments of a church (whether Methodist, or Anglican or Roman Catholic) outside of the Communion.
We cannot know, and it is not a concern for us on a formal level.
Obviously, some individuals will proffer a personal opinion. I still cross myself when I pass a Roman Catholic parish church and some Anglican parishes, and I happen to personally believe that there is something worthwhile about the sacraments and liturgies there. I believe in the Real Presence and, for what it’s worth I believe that it is there. This is something I do not do when passing most Protestant churches.
Other Orthodox may not share my opinion and rather think it is all nothing. But the church does not formally take either position.
When a potential convert approaches for acceptance into the Orthodox church that person will be required to refute any errors taught by the former church. Then the bishop must decide through what means the new Christian will be received. This usually will require economy on his part. Much like sacraments given in the Roman Catholic church “conditionally”, the method chosen by the bishop is expected to make up for any unknown possible deficiencies.
When I became Orthodox I was received with Protestants and a fallen-away Roman Catholic who never had Confirmation. We were all received by the same method, Chrismation (which is Confirmation). That does not mean the church viewed all of our religious backgrounds the same way, the church doesn’t know, the Chrismation is like spiritual medicine.
If my bishop had asked that I submit to baptism I would have readily done so, because he was my Orthodox bishop and I wanted to be Orthodox.
BTW, in Orthodoxy Chrismation is a repeatable sacrament. In other words accepting Chrismation is not in itself a de facto judgment on any previous Chrismations one has had. I have noticed that seems to be a part of the Tradition that Byzantine Catholics have lost to Latinization.