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Email security question
Tony Wright verbally sodomised in
In message , Graham
Ordinarily, a user has an email address and an internet connection that
match: for example,
The user sends outgoing email to the Demon smtp server -
smtp.demon.co.uk - which relays for him.
Not that it matters, in this context, but it would be post.demon.co.uk
So why mention it then?
Some recipients will check that the domain
does exist and the IP address is one of a the range
registered to Demon - if this check fails the email might be rejected.
No. They may check that post.demon.co.uk is who is connecting and their
DNS/rDNS is valid. They may also check that post.demon.co.uk is not in
Some users buy their own domains and email hosting from a third party:
for example . These users will normally send email
using the smtp server provided by their hosting service - something like
No. Normally it will be smtp.hosting_service.com or
relay.hosting_service.com and it will probably require SMTP
Authorisation, or similar, before it works. Since the hosting service
provider doesn't have a way to verify your credentials by your
connection, they will generally require some other form of
In the past such users may have handled email for
other people, so may wish to send emails of the form
through the smtp.mydomain.co.uk server. Most hosting services now
prevent this, since it can allow spammers to take over their smtp server
as an anonymous relay. If the user can demonstrate the legal right to
use then the hosting service might allow the user
to send through their relay.
Yes SMTP-Auth or similar
A similar situation may be true of ISPs. A user may have
but wish to send through his ISP's email server
smtp.any-isp.com - and the ISP might allow it.
Yes. Because the ISP knows the user is connected to the Internet via
them and can cut them off if they do dumb things.
But what happens to that email when it reaches its destination? The
recipient's email server sees the email coming from .
The recipient's email server sees a message coming from the ISP's
smarthost or from the hosting server's smarthost
The DNS lookup should succeed - but the smtp server that sent the email
will bear no relation to @HisDomain.Co.Uk - because it might be
smtp.mydomain.co.uk or smtp.any-isp.com - so is the email accepted or
Any DNS/rDNS/Blacklists will be done on the smarthost, not the sender
email address or domain. Of course a recipient rule may block messages
from a particular sender but I don't think that's what you're asking.
Can anybody explain what actually happens, and suggest whether any
further tightening of security might occur?
What happens at the other end can be complicated but has little to do
with . as far as DNS/rDNS is concerned.
So you don't know either.
f i l
S o n o
i u e n
g r s g
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