System.Net.SmtpClient uses Dns.GetHostName() to get the DNS name it uses for HELO.
On Windows (MS.NET) and Mono on Debian, this returns only the first part of the machine's domain name - but the name for HELO should be a FQDN.
An an example:
Dns.GetHostName() returns "test"
SmtpClient should use Dns.GetHostEntry(Dns.GetHostName()).HostName to ensure it has a valid FQDN. This returns the correct full FQDN on MS.NET and Mono.
I've submitted a pull request: https://github.com/mono/mono/pull/2020
This pull request was merged
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