SRS support

[avonwyss]

Versions

• Exchange Version: 2013-2019

Description

This is a feature request for extending DKIM signer to do a little bit more. It may be seen as out of scope since it's only loosely related to DKIM (in fact it is more about SPF, but also part of the SPF/DKIM/DMARC family). So if you think this does absolutely not belong here then go ahead and close it. Also I'm aware that currently the project seems to be in maintenance mode and that this may never materialize.

Many users who are using SPF, DKIM and maybe also DMARC may not be really aware that Exchange is fundamentally broken in the way it forwards messages, due to the fact that Exchange does support SPF, but it does not implement SRS (Sender Rewriting Scheme). In the early days of SPF, when SPF was not widely adopted and often misconfigured, admins were usually not configuring message rejection on SPF Fail results. Also, problems with legitimate mails typically only happen when forwarding to external e-mail addresses is used at the Exchange server level.

Nowadays SPF if widely deployed and getting such forwarded messages rejected or classified as SPAM is very common. Microsoft did also realize this and they implemented SRS for their online offerings, but they have not added proper SRS functionality to the On-Prem Exchange versions (it's all only about Exchange Online, see here, here and here).

So my idea is that code could be added to DKIM Signer so that messages would be rewritten for SRS when necessary, as to improve deliverability on Exchange On-Prem.