Marriage between the baptized is indissoluable. It is for life. Jesus tells us that those who divorce their (valid) spouse and remarry another commit adultery. So, a Catholic who divorces is not able to marry another while their spouse is alive.
The Church has a process for examining marriages that have what is called an impediment. This means a valid marriage did not actually take place because one or more impediments existed at the time of the marriage.
The Church does not annul a marriage-- make it invalid-- the Church investigates marriages and if it finds that it was invalid from the start issues a finding of that fact.
I can suggest the book Annulment: The Wedding That Was by Michael Smith Foster to help you understand this topic. It can be quite complex.
I don’t know why you have never heard of this. It is quite common knowledge that divorce and remarriage are sins-- even among non-Catholics. Although, many non-Catholic denominations ignore this particular teaching of Our Lord.
I am not sure who emailed who or why. That is a matter for the Catholic to take up with his/her pastor.
A Catholic who is divorced and remarried may not receive communion. That is because they are committing adultery.