How can I fix email confirmation - in .NET Core, it doesn't work

Question

I already have a register action

public async Task OnPostAsync(string returnUrl = null)
{
            var useremail = _userManager.User         


        

Answer 1

"Solved" I asked Sendgrid, and I was told that I cannot use my yahoo email (or gmail,...) as the sender email; this is part of the answer: "Yahoo observes an email security standard called DMARC. DMARC instructs email providers to reject messages where the From domain is a Yahoo domain, but the message originates from a non-approved domain server/service." So I need to use my own mail domain;