how to send an email?

[Maxim Gorbunov]

А DNS-сервер указывать пробовали.

Цитата:

IMPORTANT: If you do not specify a DNS server before calling this method then it will take a substantial amount of time to perform the validation. Make sure you use a DNS server!

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Цитата:

ValidateAddress uses the DNSServers collections to perform mail exchange lookups, so it is highly recommended that you add multiple DNS servers to this collection. If no DNS servers have been added to the DNSServers collection then the following default servers will be tried in their respective order: "A.ROOT-SERVERS.NET", "B.ROOT-SERVERS.NET" and "C.ROOT-SERVERS.NET". If these servers are not available then the method call will fail and return an error code.