Whatever you do under the esa.XX domain, postmaster@esa.XX *must* be a valid address.
Who tells ? Just because it's a domain name it need not be a valid (domain part of a) mail address - unless there is an MX. Have you ever tried postmaster@ac.uk ?
For the hypothetical case where this would happen, the question remains for me if and how the responsibility for a 2nd-level subdomain can be shared and coordinated between two organisations? A domain "belongs" to one specific organisation; subdomains under it cannot "belong" to or be shared with a completely different organisation.
Again, who "owns" ac.uk ? ESA.XX could be the common 2nd level domain for all organisations with acronym ESA. It's a local decision to associate 2nd level domains with organizations.
Crazy idea: if company X moves from Cologne to Darmstadt, its domain name would change, they would have to reprint their letter heads, business cards, etc., and someone would have to cope with all the bounces of mails sent to the old domain name... Totally correct. If the company moves, they will have to change their business cards, letter heads ...
-Peter