Error e-mails automatically generated by mail servers' MTAs usually appear to have been sent to the postmaster address.
The rfc-ignorant.org website used to maintain a list of domains that do not comply with the RFC based on this requirement, but was shut down in November 2012.
This postmaster address is not strictly necessary if the server always returns 554 on connection opening (as described in section 3.1).
In extreme cases (such as to contain a denial of service attack or other breach of security) an SMTP server may block mail directed to Postmaster.
However, such arrangements SHOULD be narrowly tailored so as to avoid blocking messages which are not part of such attacks.