Re: [PATCH RFC] send-mail: add support for Microsoft Graph API

> Apart from SMTP, Microsoft also provides a REST API, branded as
> Microsoft Graph for sending mails. Upon testing a bit, I have
> found a few benefits over SMTP. Firstly, SMTP servers of Microsoft
> are kinda slow. On an average, initialising the SMTP server even
> on a fast internet connection takes around 8-10 seconds with
> send-email. Once initialised, subsequent messages sometimes also
> face delays, taking around 3-5 seconds per message, and other
> times they are sent almost instantaneously. Secondly, their SMTP
> server does not respect the Message-ID specified by the user and
> replaces it with their own generated string.
> 
> Microsoft Graph API solves both these problems. It is extremely
> fast, taking around 1 second to send a series of 5 patches, and
> also respects the Message-ID specified by the user.
> 
> After this patch, users can use the graph API by having their
> config as:
> 
> [sendemail]
> 	useMSGraph = true
> 	MSGraphUser = yourname@outlook.com
> 
> The API requires an OAuth2.0 access token for authentication, and
> users can either generate them manually, or use a credential helper