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Unable to send domain.

We cannot send to this one domain for ours. If an email is sent then and instant email gets returned saying "Delivery has failed to these recipients or distribution lists".
This is also displayed under the email being sent to: Requested #550 5.5.0 Invalid EHLO/HELO domain. ##

A message was sent to the domain in question from a hotmail account and it got through fine.

Can anyone help with this?
3 Solutions
Alan HardistyCo-OwnerCommented:
Have a read of this article - not the best written article in the world, but it should explain why this might be happening and what to do about it:
Rajith EnchiparambilOffice 365 & Exchange ArchitectCommented:
Make sure you have a reverse dns record for your email ip address (MX).

Make sure that the banner matches the MX record url, mail.domain.com

Use http://www.mxtoolbox.com/diagnostic.aspx to check your existing settings.
"Requested #550 5.5.0 Invalid EHLO/HELO domain"

When SMTP server initially connect, they announce themselves (this is called a HELO) giving the server name, according to the SMTP RFC this address has to be a VALID resolvable DNS names. Most people set this to the same name as their PTR record. Chances are your outgoing server is announcing itself as some internal active directory domain name (exch.yourad.local) or a non-qualified name (exch).

For Sendmail, you just have to edit the /etc/hosts file to change that name.

For Exchange (2003) there is a "fully-qualified domain name" on the advanced section of the delivery tab on the properties for the SMTP virtual server.
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