El kéne olvasni azt a fentebb linkelt rfc-t (4.1.2):
HELO <SP> <domain> <CRLF>
...
The syntax of the above argument fields (using BNF notation where applicable)
is given below. The "..." notation indicates that a field may be repeated one
or more times.
...
<domain> ::= <element> | <element> "." <omain>
<element> ::= <name> | "#" <number> | "[" <dotnum>"]"
<dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
<snum> ::= one, two, or three digits representing a decimal
integer value in the range 0 through 255
...
Azaz szintakszisát tekintve a "helo [1.2.3.4]" is helyes.