It is possible to delegate a subdomain to external nameservers (if this subdomain is setup on the server corresponding to the set nameservers), according to http://zytrax.com/books/dns/ch9/delegate.html.

I am wondering whether the same is achieveable with e-mail.

So, is it possible to add DNS entries so that any e-mail send to a subdomain will delegate to the "e-mail server" of an external server, without needing my server to actually process the data?

If so it would be possible to create a service where people can register their own e-mail aliases.

Thanks in advance.

Yes. The example on the zytrax site does just that:

; zone fragment for sub-domain us.example.com
…
; sub-domain mail server
                  IN      MX 10  mail.us.example.com.

_________________
/ Peter

It may or may not be obvious, but if you have delegated a subdomain to someone else's DNS servers, then THEY will be the ones who can set the MX for that subdomain, not you.

If you haven't delegated the subdomain, then a simple MX entry for the subdomain should do what I think you are wanting.