private void SendMail()
{
    var usersEmailAddresses = Factory.Users.List(); // .List() lists all the data from the Users table.

    Application OutlookApplication = new Application();
    MailItem OutlookMail = (MailItem)OutlookApplication.CreateItem(OlItemType.olMailItem);

    OutlookMail.Subject = @"TEST/Ushqimi i caktuar per sot";

    MailBody //region, here I assign the MailBody Text.

    for (int index = 0; index < usersEmailAddresses.Count; index++)
    {
        OutlookMail.To = usersEmailAddresses[index].Email;  //Here is where I get the exception, AFTER trying to assign the second email.

        if (usersEmailAddresses[index].RecieveEmail && !usersEmailAddresses[index].IsOnVacation)
        {
            ((_MailItem)OutlookMail).Send();
        }
    }
}

MailItem OutlookMail = (MailItem)OutlookApplication.CreateItem(OlItemType.olMailItem);