The parser had to fit entirely within a 16-bit address space (alongside the rest of the program) so the syntax was designed to allow that.

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

as a side project by an INGRES developer in 1979 to connect to Arpanet.