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Probably ok from a 'verify the syntax' approach. Since mx records are just a pointer to a server, there is no guarantee that any of the listed machines will answer on port 21, or that they would deliver mail to the intended recipient.
Port 21 is FTP. Did you mean port 25 (SMTP)? If not, I'm missing your point.
Keep in mind also that DNS servers can fail. If there is any sort of routing problem in the internet, you might not receive a response, so you may want to consider treating a failure as a warning.
That's why you're always required to have two DNS servers when you host a domain, so that even if one goes down, you can access the other one. Granted, in some very unusual circumstances, you might get cut off from both of them (particularly true when it's YOUR connection that's down) but that's not very common.
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