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Re: SMTP Invalid Address (501 5.5.4)

From: Dan Fandrich <>
Date: Thu, 22 May 2014 08:18:09 +0200

On Wed, May 21, 2014 at 08:38:22PM +0100, Steve Holme 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. 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.

>>> Dan
List admin:
Received on 2014-05-22