Don't try to reatedly deliver when remote doesn't support smtputf8

[jnd-cz]

In my recent round of testing with non ASCII characters I found that Mox tried to attempt multiple deliveries when the remote side doesn't support smtputf8. I don't think it's transient error, the other server won't suddenly start supporting it so one attempt should be enough.

Delivery has been delayed of your email to:

    [ping@tools.mxtoolbox.com](mailto:ping@tools.mxtoolbox.com)

Next attempts to deliver: in 4 hours, 8 hours and 16 hours.
If these attempts all fail, you will receive a notice.

Error during the last delivery attempt:

    remote smtp server does not implement smtputf8 extension, required by message, transient

[mjl-]

I think this issue has come up before. I think I originally put it in because openbsd spamd, used for greylisting, i.e. first-time connections are sent to spamd, did not announce smtputf8, but the actual mail server did. At least openbsd spamd has been fixed in the mean time (though existing setups may still be running the old version, so the behaviour hasn't gone away completely, likely).

Perhaps that was the only case where it was relevant... However, users won't be happy if messages that could be delivered in the past (to certain destinations) no longer can't.

Perhaps a better tradeoff is to limit the number of retries to max 1 hour or so. I think greylisting doesn't usually last longer than that.