What does the message retry time not reached [for any host] on the log mean? Why won't Exim try to deliver the message?

That is not an error. It means exactly what it says. A previous attempt to deliver to that address failed with a temporary error, and Exim computed the earliest time at which to try again. This can apply to local as well as to remote deliveries. For remote deliveries, each host (if there are several) has its own retry time. If you are running on a dial-up host, the rest of this answer probably does not apply to you. Go and read `FAQ/Dialup_and_ISDN/Q1404`_ instead. If your host is permanently online, read on...

Some MTAs have a retrying schedule for each message. Exim does not work like this. Retry timing is normally host-based for remote deliveries and address-based for local deliveries. (There are some exceptions for certain kinds of remote failure - see Errors in outgoing SMTP in the manual.) If a new message arrives for a failing address and the retry time has not yet arrived, Exim will log retry time not reached and leave the message on the queue, without attempting delivery. Similarly, if a queue runner notices the message before the time to retry has arrived, it writes the same log entry. When the retry time has past, Exim attempts delivery at the next queue run. If you want to know when that will be, run the exinext utility on the address, for example:

exinext user@some.domain

You can suppress these messages on the log by including -retry_defer in the setting of log_selector. You can force a delivery attempt on a specific message (overriding the retry time) by means of the -M option:

exim -M 10hCET-0000Bf-00

If you want to do this for the entire queue, use the -qf option.