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This uses fetchmail to issue an etrn command that causes the MTA on the secondary mail server to process the queue for the specified domain.
You can also just use telnet or nc to connect to port 25 of the server and then:
250 2.0.0 Queuing for node yourdomain.example.org started
Using perl, here, we grep the man page of fetchmail to find the paragraph starting with '-k | --keep' and ending before the paragraph starting with '-K | --nokeep'