How to retrieve non-delivered postfix emails?

In principle, if you have something in the queue, you may find a list of such mail via

postqueue -p

which tells you IDs. Then, for each ID, you may print a complete body of that email with

postcat -q <ID>

And if you decide to delete a message,

postsuper -d <ID>

However, in your case you have nothing in the queue, so nowhere you can get the mail, unless it was saved somewhere before it was given to Postfix.

Your mail was rejected with 554 code. 5xx codes mean non-recoverable error. This kind of error makes Postfix think the mail is not worth keeping, because it could not be delivered in any subsequent attempts. By the way, the "domain not found" should not be "non-recoverable" (because it could be the problem with the DNS on the receiver side), but in this case you seem to have "private" DNS name, which could justify the hard fail.