Certain MTAs (other than sendmail) do not return immediately on RCPT of undeliverable addresses and exacerbate the spam backscatter problem.

it was a simplification of X.500, the directory service from OSI whose X.400 was a competing standard to SMTP (and also used ASN.1)

@,!, and : each had different right/left associativities which made them a real pain to parse and rewrite properly.

A book was published called “!%@:: A Directory of Electronic Mail Addressing and Networks” was published