"On Behalf of"/Sender header on e-mails for better deliverability

[JonGold]

I have a client, an umbrella organization for a number of regional affiliate groups.  They recently came to CiviCRM from Salsa.

One feature that Salsa has that CiviCRM does not is the addition of a "Sender" mail header.  When sending mail for jon@nonprofit.org, instead of the header:

Code: [Select]

From: "Jon" <jon@nonprofit.org>
The headers are:

Code: [Select]

From: Jon
Sender: jon=nonprofit.org@mail.salsalabs.net
Reply-To: jon@nonprofit.org

This means that Salsa users don't need to account for Salsa in their SPF records.

This seems to be to be a worthwhile addition to core - both for multi-site orgs and for SaaS-based Civi (like Cividesk) as a way to simplify migration.

My client is willing to provide funding for implementation, but I'd love some feedback on the most welcome approach:
* Is this appropriate for core, or should it be an extension?

If so, then:
* Should this setting be per site? Or per "From" address?  The latter is tricky because "From" addresses are option values. that's an option group.
* Should there be an option to turn this off on a per-mailing basis?  The setting itself can't be per-mailing, because there needs to be a default for non-CiviMail mailings.

[JonGold]

As a follow-up - we can get 98% of what we want by using the CiviMail option to allow a custom Reply-To.  It's not quite as nice as the approach above, but it does make this a much lower priority.