The tcp/ip abort interval in Solaris 2.x is too short,
the default value is 2 minutes. The result is that when an
ACK isn't received in 2 minutes, the connection is closed.
This is most often seen by sendmail, which will log
sendmail: SYSERR: collect: read timeout on connection from ...
You can fix this by running following command which increases
the timeout to 8 minutes (unit is millisec), which is the
Solaris 2.4+ (and patched 2.3) default.
/usr/sbin/ndd -set /dev/tcp tcp_ip_abort_interval 480000
This command should be placed in a script rc2.d script. (See 3.20)
(See 5.21 for another possible cause)