4 ms·

Folks discussing this are knowledgeable about the SMTP protocol. So let me ask something I have always wondered about -- what was the idea behind HELO, what pur

by srean 6y ago

Folks discussing this are knowledgeable about the SMTP protocol. So let me ask something I have always wondered about -- what was the idea behind HELO, what purpose does it serve in the protocol. Thanks to all who take out their time to answer this 'ought to be obvious' question.

megous 6y ago
> the command may be interpreted as saying "Hello, i am <host>".
- srean 6y ago
  Indeed but what's the purpose of that. The SMTP server knows the IP address anyway and can DNS lookup if needed. The one chatting with server can say hello followed by any arbitrary string
  - ink_13 6y ago
    > can DNS lookup if needed Can it? Not every machine that sends mail is on the public Internet.
  - quesera 6y ago
    SMTP (1982) predates DNS (1985). Reverse DNS was added to SMTP servers as an anti-spam feature about 15 years after HELO was established. But to be honest, I'm not sure HELO was ever really useful. In the very early years, it might have been appropriate to reject messages where the MAIL-FROM domain did not match the HELO domain. Not for long though.
    - albertgoeswoof 6y ago
      This is the crazy thing about email, it's ancient, 39 years old. I bet it'll still be around in 20 years from today.
    - srean 6y ago
      Ah! That was the missing information. Thanks, now it makes sense
  - megous 6y ago
    HELO is not really used that much anymore. It's EHLO now. And response to it includes capabilities of the server. So it serves negotiation of those at the beginning of the session, too. It's better to be prompted for those, instead of just sending them unannounced, in case client just want to switch to TLS first via starttls.
jeffbee 6y ago
SMTP is an interesting protocol because the server opens the session with the banner, unlike HTTP where the client speaks first. I think a better question is why does the banner exist, and what purpose does it serve? HELO/EHLO gives us a way to negotiate ESMTP, which is what practically everyone uses today.

by srean 6y ago

megous 6y ago

> the command may be interpreted as saying "Hello, i am <host>".

srean 6y ago
Indeed but what's the purpose of that. The SMTP server knows the IP address anyway and can DNS lookup if needed. The one chatting with server can say hello followed by any arbitrary string
- ink_13 6y ago
  > can DNS lookup if needed Can it? Not every machine that sends mail is on the public Internet.
- quesera 6y ago
  SMTP (1982) predates DNS (1985). Reverse DNS was added to SMTP servers as an anti-spam feature about 15 years after HELO was established. But to be honest, I'm not sure HELO was ever really useful. In the very early years, it might have been appropriate to reject messages where the MAIL-FROM domain did not match the HELO domain. Not for long though.
  - albertgoeswoof 6y ago
    This is the crazy thing about email, it's ancient, 39 years old. I bet it'll still be around in 20 years from today.
  - srean 6y ago
    Ah! That was the missing information. Thanks, now it makes sense
- megous 6y ago
  HELO is not really used that much anymore. It's EHLO now. And response to it includes capabilities of the server. So it serves negotiation of those at the beginning of the session, too. It's better to be prompted for those, instead of just sending them unannounced, in case client just want to switch to TLS first via starttls.

jeffbee 6y ago

SMTP is an interesting protocol because the server opens the session with the banner, unlike HTTP where the client speaks first. I think a better question is why does the banner exist, and what purpose does it serve? HELO/EHLO gives us a way to negotiate ESMTP, which is what practically everyone uses today.