Re: [PATCH] send-email: add --get-smtp-server option to fetch SMTP settings

> Autoconfiguring SMTP server settings is a common feature present in many
> email clients. In order to get the correct SMTP server settings easily,
> this commit adds a `--get-smtp-server` option to `git send-email`. This
> option attempts to fetch the SMTP server settings for a given email address
> via the following steps:
> 
> 1. It first tries to fetch the settings from Mozilla's ISPDB at
>    `https://autoconfig.thunderbird.net/v1.1/[domain]`.
> 
> 2. If that fails, it attempts to fetch the autoconfig file from the email
>    provider's autoconfig URL, which is typically in the format
>    `https://autoconfig.[domain]/mail/config-v1.1.xml?emailaddress=[email]`.
> 
> 3. If that also fails, it falls back to checking the MX records of the
>    domain used in the email address to find the SMTP server. It can be
>    useful in case of emails with custom domains. It attempts to guess
>    the correct domain for the email from the MX records, and repeats the
>    first 2 steps with the guessed domain.
> 
> This feature is heavily inpired by the autoconfig feature in Mozilla
> Thunderbird. A detailed documentation about how thunderbird fetches the
> autoconfig settings can be found at:
> 
> https://www.bucksch.org/1/projects/thunderbird/autoconfiguration/