%% You should probably cite rfc1846 instead of this I-D. @techreport{ietf-mailext-smtp-521-03, number = {draft-ietf-mailext-smtp-521-03}, type = {Internet-Draft}, institution = {Internet Engineering Task Force}, publisher = {Internet Engineering Task Force}, note = {Work in Progress}, url = {https://datatracker.ietf.org/doc/draft-ietf-mailext-smtp-521/03/}, author = {Francis Dupont and Alain Durand}, title = {{SMTP 521 Reply Code}}, pagetotal = 4, year = 1995, month = mar, day = 23, abstract = {This memo defines a new SMTP ({[}1{]}) reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail.}, }