From IBM :
Why is sendmail logging « gethostbyaddr(IPv6:::1) failed: 1 » warning messages to syslog?
In AIX 5.3 TL11 and AIX 6.1 TL4 and later, sendmail is IPv6 enabled. When sendmail attempts to resolve local interfaces, it will encounter the IPv6 loopback interface (::1) and perform an IPv6 lookup, which fails and thus the gethostbyaddr warning is logged to syslog.
To resolve this matter, add this entry into the /etc/hosts file
hostent -a ::1 -h "loopback localhost"
Future releases of AIX will automatically include this entry in the /etc/hosts file.
Also, add the following entry to /etc/netsvc.conf :