[Exim] Error message that I don't understand

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What would an error message in the log that looks like this:

2003-03-10 08:18:33 H=mailrelay.icubed.com [208.0.145.10]:2023
F=<inv@???> temporarily rejected RCPT <lgm@???>: Could not
complete sender verify callout

Mean?!?!


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