During registration flows, I'm also a fan of validating that the domain has MX records published - DNS queries are a pretty cheap option for detecting whether i

During registration flows, I'm also a fan of validating that the domain has MX records published - DNS queries are a pretty cheap option for detecting whether its at least plausible that your validation email will arrive later. At scale, its an easy option for helping users detect typos in their email domain (or for helping guide less technical users, like those who might provide address@gmail or similar instead of address@gmail.com)