ConcurrentBag<Participant> participantsList = new ConcurrentBag<Participant>() {
    new Participant() { EMail = "Test.Mail@outlook.de", FirstName = "First1", LastName = "Last1"},
    new Participant() { EMail = "Test.Mail@gmail.de", FirstName = "First2", LastName = "Last2"},
    new Participant() { EMail = "Test.Mail@yahoo.de", FirstName = "First3", LastName = "Last3"},
};

Parallel.ForEach(participantsList, (p) =>
{
    var mail = new Email("REGISTERMAIL", p.FirstName, p.LastName);
    mail.AddRecipient(p.EMail);
    mail.Send();
});