Why don’t most ESP forwarders use SRS?
The current list of challenges is correct but the primary reason is strategic. SRS is a complex solution for an obsolete problem.
It was created to fix SPF forwarding in an SPF centric world. The modern email ecosystem relies on DKIM alignment which is more flexible survives forwarding and is far easier for vendors to support. It is simpler to implement aligned DKIM than to deploy SRS which only fixes SPF.
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