RE: 550 Error: Helo command rejected: Host not found
- From: Stu <Stu@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 27 Apr 2005 14:13:28 -0700
You know...I may be able to solve this by changing the FQDN for the Exchange
Virtual SMTP server from the machine name to mail.example.com. Other email
servers would check against their DNS to verify that the email from
example.com is valid and it would resolve appropriately to my existing
mail./MX entry.
If folks have thoughts or definitive solutions I'm interested!
"Michel Boks" wrote:
>
> Well, Reverse DNS is a bit tricky, since there can be multiple domains on 1
> IP address and an IP address can have only one PTR record. That's why most
> companies don't use this but other filters.
> Remember that the MX and A records work the other way than the PTR record.
> You might test if your IP address has a registration by using ping -a from an
> external machine.
>
> (My company almost 'lives' on mail, we had this issue only once, since then
> we asked our ISP (one of the biggest Telco's here) to create the PTR record.
> No problems since then)
>
> Regards,
> Michel
>
> "Stu" wrote:
>
> > I'm thinking that something along these lines is the cause of this new error
> > when communicating with this specific client. In fact, I think the likely
> > resolution will be to create an additional A record for my server's name
> > (server.example.com).
> >
> > We already have an A record for mail.example.com and the MX records reflect
> > this appropriately. I find it interesting that this is the first time that
> > I've seen this 550 error as one would assume that at least one other mail
> > server is checking for reverse DNS.
> >
> > I'm also curious to know if there's a way to change the Received header info
> > for email that is generated within my domain. I see a Masquerade Domain
> > setting within Exchange 2000 that has captured my interest.
> >
> > "Michel Boks" wrote:
> >
> > > Stu,
> > >
> > > Any chance your client is using Reverse-DNS checking now and that your
> > > hostname isn't registered to that IP address (as PTR record at the ISP)?
> > >
> > > Regards,
> > > Michel Boks - B.ICT
> > > MCSE NT4/2000
> > >
> > >
> > > "Stu" wrote:
> > >
> > > > A user received this message in their Outlook email when attempting to send
> > > > to a specific client of ours. Let's say that my server's name is
> > > > server.example.com - the DNS entry for my mail server is mail.example.com...
> > > >
> > > > server.example.com #5.5.0 smtp;550 <server.example.com>: Helo command
> > > > rejected: Host not found>
> > > >
> > > > We have been able to successfully email our client in the past so I think
> > > > that they've rececenly added some security to their system. I think what is
> > > > happening is their mail server is checking the host that is sending
> > > > (server.example.com) and not finding anything in DNS that matches that.
> > > >
> > > > Do I need to make an entry in DNS for server.example.com or is it possible
> > > > to change the header info being populated by Exchange so the Received: from
> > > > equals mail.example.com?
> > > >
> > > > Any help is appreciated. I've looked for answers but most have to do with
> > > > Postfix or sendmail servers.
.
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