Re: [exim] Routing failed deliveries through an ESP

On 17/04/2023 14:08, Bill Cole via Exim-users wrote:
> There's a rational basis for an exception for 5xx before MAIL FROM, when the target only has the connection parameters and HELO name to use as a basis for rejection. Re-routing via a fallback path isn't entirely unjustifiable in that case, as it changes those elements of the transaction.

Exim treats what you're talking of as a "host error" rather than a "message error",
and goes on to try the next host in the list of possibles determined by the routing
stage. Commonly that would be a lower-priority MX for the domain.
--
Cheers,
Jeremy