Send-MailMessage Excluded from Non-Windows systems

[tibmeister]

Not sure of the background on this one but if it's something that just needs re-written and tweaked for non-Win systems then I could probably take that on. I can't even count the number of scripts I have that rely on this.

[joeyaiello]

Looks like it depends primarily on System.Net.Mail.

Fortunately, it also looks like we'll get some semblance of Mail with .NET Standard 2.0

[ThreeFive-O]

@joeyaiello I think this issue can be closed as the Send-MailMessage cmdlet was ported in PR #3869