I should clarify this for the benefit of folks who aren't deep into email.
By "temporary failures" I mean 4xx response codes in SMTP - codes that will
tell the client "this won't work now, try again later". As opposed to
permanent (5xx) failures which tell the client "this won't ever work" and
cause the client to bounce the message.
I don't think this will fly. Any period of time during which valid mail will
be bounced is too long. Brief periods of time during which temporary failures
would result from a mismatch might be tolerable, but a day is too long.