You can use ARSoft.Tools.Net library to get the available SMTP servers for domain.
It's available on NuGet Packages:
PM> Install-Package ARSoft.Tools.Net
Import the namespace:
using ARSoft.Tools.Net.Dns;
Then make a synchronous lookup as:
var response = DnsClient.Default.Resolve("yourdomain.com", RecordType.Mx);
var records = response.AnswerRecords.OfType<mxrecord>();
foreach (var record in records) {
Console.WriteLine(record.ExchangeDomainName);
}
</mxrecord>
Which will give output like :
mail1.yourdomain.com
mail2.yourdomain.com
mail3.yourdomain.com
mail4.yourdomain.com
Full documentation is available
Here[
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