register esa.ch

> Whatever you do under the esa.XX domain, postmaster at 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 at 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