550 5.7.1 Relaying denied. IP name lookup failed

550 5.7.1 Relaying denied. IP name lookup failed

Recently we configured a BigIP virtual for SMTP called smtp.justanexample.com

Whenever I tried sending mail through this it would complain:

# telnet smtp.justanexample.com 25


Trying 192.168.10.201...

Connected to smtp.justanexample.com
Escape character is '^]'.

220 smtp1.justanexample.com ESMTP Sendmail 8.13.8/8.13.8; Mon, 15 Apr 2013 22:34:54 -0400

mail from:vikram.das@justanexample.com

250 2.1.0 vikram.das@justanexample.com... Sender ok

rcpt to:vikram.das@justanexample.com

550 5.7.1 vikram.das@justanexample.com... Relaying denied. IP name lookup failed [192.168.10.2]

Doing an nslookup on this name failed.  If I did https://192.168.10.2 it would go to the BigIP user interface.  This IP address was somewhere related to the BigIP device, and did not have a hostname in DNS.  I have sent a mail to the network team for further investigation about this IP.

The usual practice for allowing relay from certain hosts is to add those hosts to /etc/mail/local-host-names.  However since this IP was not present in DNS, I added the following line to /etc/mail/access on all smtp nodes :

Connect:192.168.10.2             RELAY

and bounced sendmail on Linux

/etc/init.d/sendmail restart

The Relaying Denied is not coming anymore.