This is a bit of an archaeological question, but RFC 821, page 31, paragraph 3 describes a syntax for mailbox addresses that is of the form
local_path@#123, where the pound and the following number is some sort of address. It doesn't mention what sort of address that is. It might have ben common knowledge at the time, but I couldn't find a reference for what sort of address that is.
Sometimes a host is not known to the translation function and communication is blocked. To bypass this barrier two numeric forms are also allowed for host "names". One form is a decimal integer prefixed by a pound sign, "#", which indicates the number is the address of the host. Another form is four small decimal integers separated by dots and enclosed by brackets, e.g., "[188.8.131.52]", which indicates a 32-bit ARPA Internet Address in four 8-bit fields.