Do you have an idea of what kind of invalid emails you are receiving? That will help you determine which vendor is the best fit for you. For example do you consider support@, info@, or postmaster@ invalid, some vendors will weed those out for you? Or are you just concerned with making sure the email has an accurate Domain/MX Record? Will blocking SPAM IP addresses be enough to address your needs, if so you need a vendor who checks agains URI DNSBLs. With that in mind, here's a good blog article about the top vendors. Best wishes! https://www.formget.com/best-email-verification-services/