[exim] A way to treat 500 authentication failures as transie…

Is there a way to have Exim treat a 500-failure on SMTP authentication with
a smarthost as a transient error, and simply re-queue the message rather
than generating an NDR?

Our smart host sometimes generates a 500 on authentication, but it's not
really a permanent error - just a temporary glitch. I don't want our users
to see NDRs in this case.

Thanks,
Ken