Is there a solution for 550 User Unknown mails

Chuck Swiger cswiger at
Wed Jan 3 16:55:09 PST 2007

Derrick Ryalls wrote:
[ ... ]
> With secondary mail servers, I am accepting mail for other domains,
> and then forwarding them on.  It is my belief that acting as a
> secondary mail server is what is causing me to receive all these
> errors.  What I believe is happening is that for whatever reason the
> spammer is talking to my email server for a domain I am secondary on.
> I accept the mail and finish the conversation.  I then talk to the
> main email host to let them know of the email I got for them.  The
> master host responds with 550 User Unknown as it should.  Being the
> good little secondary email server I am, I try to respond to the
> sender that they got the wrong address, which turns out to be fake.
> Assuming that my guessing is correct, is there any way to stop this
> flood of reject emails short of not being a secondary server for
> anyone?

Your assumptions are basically right.  The best way to deal with this is to 
make the secondary MX you run aware of which email addresses are valid and 
which are not, so it can reject misaddressed mail rather than accepting it and 
then having to bounce it later.  Setting this up via LDAP, NIS, or whatever 
might be possible but it typically is difficult.

Intermediate solutions include using greylisting or creating a manual 
blacklist of frequently-spammed invalid accounts which you reject immediately.


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