We get an issue where messages intermittently fail with a "550 Unauthenticated senders not allowed". We're working on getting to the bottom of that, but in the meantime, I want to configure exim such that when messages fail with that error, they go into a queue to be automatically retried (since the error is intermittent, they may work fine a few minutes later so that'd be a fine workaround in the meantime).
Currently what happens is they bounce and get frozen. If I thaw one of the messages, it's not the original message, but the bounce message.
Is this possible?
This topic has been discussed a lot on the Exim mailing list, and it cannot do what you want, nor will it ever become possible; a permanent error is just that: permanent. All 5xx errors are permanent.
If your third party e-mail service is prone to this problem, then they should fix their MTA to report an 4xx error when this situation occurs.
If the bounce messages become frozen, then you don't have a valid envelope sender on your outgoing messages, which is something that helps to get your messages marked as spam; so you need to fix that.
You can configure the server that is giving you the 550 to send a 450 (or something) instead.