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RE: SMTP Invalid Address (501 5.5.4)

From: Steve Holme <>
Date: Thu, 22 May 2014 08:00:55 +0100

On Thu, 22 May 2014, Dan Fandrich wrote:

> > My understanding is that the SMTP server can validate what you send
> > it but, as long as it looks like a domain then the server shouldn't
> > reject it if what is mentioned in RFC 1123 section 5.2.5 is correct.
> > It can't, for example, perform a reverse DNS look up, check if the IP
> > address matches the address that you're connecting from and then
> > reject your connection because they are different. It can however,
> > then reject the email when you specify the MAIL or RCPT commands
> > or set SCL headers etc... etc..
> "Can't" is a bit harsh.

Sorry - I appreciate I don't always select the right words but I didn't want
to use "shouldn't" twice in close successful ;-)

> The RFCs might not suggest or allow it, but some hosts do it
> nonetheless. The mail host for, for example, used to do
> some pretty fascist checking. I wasn't able to send mail to that host
> for years from any of my personal e-mail addresses because it rejected
> connections for all kinds of specious reasons, starting with an invalid
> host after EHLO. It seems to be less harsh today, but don't discount mail
> hosts severely bending the RFCs in the name of spam prevention.

I understand that programmers, myself included, don't always read spec's and
when we do, we don't always follow them. I also appreciate that there is
nothing to stop anyone doing something different to what is written or
recommended in an RFC however, the RFC reads:

"The HELO receiver MAY verify that the HELO parameter really
corresponds to the IP address of the sender. However, the
receiver MUST NOT refuse to accept a message, even if the
sender's HELO command fails verification."

As you may have noticed I misinterpreted it, as I thought, the connection
couldn't be refused but the message could :(

Kind Regards

List admin:
Received on 2014-05-22