`System.Net.Mail.SmtpClient' is obsolete

[mans0954]

Hello,

I'm trying to build with mono 4.6.2 on Debian, by running the command:

xbuild /p:Configuration=NET45-Release /t:RunAllTests buildscripts/Build.proj

This fails with

Build FAILED.
Errors:

/home/mans0954/2017/Core-4.0.0/buildscripts/Build.proj (RunAllTests) ->
(BuildProject target) ->
/home/mans0954/2017/Core-4.0.0/Castle.Core.sln (Build) ->
(Build target) ->
/home/mans0954/2017/Core-4.0.0/src/Castle.Core/Castle.Core.csproj (default targets) ->
/usr/lib/mono/xbuild/14.0/bin/Microsoft.CSharp.targets (CoreCompile target) ->

	Core/Smtp/DefaultSmtpSender.cs(233,47): error CS0618: Warning as Error: `System.Net.Mail.SmtpClient' is obsolete: `SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead'
	Core/Smtp/DefaultSmtpSender.cs(267,11): error CS0618: Warning as Error: `System.Net.Mail.SmtpClient' is obsolete: `SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead'
	Core/Smtp/DefaultSmtpSender.cs(159,5): error CS0618: Warning as Error: `System.Net.Mail.SmtpClient' is obsolete: `SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead'
	Core/Smtp/DefaultSmtpSender.cs(176,6): error CS0618: Warning as Error: `System.Net.Mail.SmtpClient' is obsolete: `SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead'
	Core/Smtp/DefaultSmtpSender.cs(272,16): error CS0618: Warning as Error: `System.Net.Mail.SmtpClient' is obsolete: `SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead'
	Core/Smtp/DefaultSmtpSender.cs(276,25): error CS0618: Warning as Error: `System.Net.Mail.SmtpClient' is obsolete: `SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead'

	 0 Warning(s)
	 6 Error(s)

[jonorossi]

Our build server has Mono 4.6.1 so I'm not sure why that warning isn't being emitted there. It looks like the Mono team added the ObsoleteAttribute to SmtpClient in Mono 4.3.2: mono/mono@fa081c5.

Happy to take a pull request to wrap DefaultSmtpSender with #pragma warning disable/restore 618, just add a comment before the disable stating that it is for Mono.

Would be great to know why our build server isn't failing at the moment though as it is running the exact same xbuild command.

[jonorossi]

@mans0954 strangely I couldn't reproduce this on Mono 4.6.1 but I've added (#271) the pragmas because I suspect we'll get these soon enough when we upgrade.