Destination mail server ... is it really not reachable?

[vaio1]

Hi guys,
I suppose that some server are not reachable for some particular mail server configuration.

I need to understand why this message appears:

This is an automatically generated Delivery Status Notification.

Unable to deliver message to the following recipients, due to being unable to connect successfully to the destination mail server.

       emailtest@flashidiomas.com.br

Can it be a problem of my mail server not configured correctly or is it a problem of the remote mail server ?

Thanks

[Hawker]

If the error doesn't happen on every mail sent, then it's not a problem with your setup.

An unable to connect error can be a multitude of things, including your IP address being blocked by a firewall rule. Personally I drop (not reject) connections from many IP addresses which would give you the error you have.

[vaio1]

How can I drop and not reject, the connections from many IP addresses ?

Thanks

[Hawker]

Directly through iptables or if you're using Bastille firewall by adding this to etc/Bastille/firewall.d/post-rule-setup.sh...

/sbin/iptables -I INPUT -p tcp -s IPADDRESS --dport PORTNUMBER -j DROP

Then /etc/init.d/bastille-firewall restart

The options for -j are ACCEPT, DROP, REJECT and RETURN (jump to another chain in iptables).

Be carefull you can block more than you want to. For example I ACCEPT mail on port 25 from howtoforge.com but DROP all other ip addresses in their CIDR (81.0.0.0/8) that attempt to access port 25.

[edge]

Could also be a bad configured DNS server.