First, you have to unpack the archive file in some safe location:
$ gunzip -c sendmail5.65b+IDA1.5+mailx5.3b.tgz | tar xvf -
If you have a ``modern'' tar, for example from a recent Slackware Distribution, you can probably just do a tar -zxvf filename.tgz and get the same results.
Unpacking the archive creates a directory named sendmail5.65b+IDA1.5+mailx5.3b. In this directory, you find a complete installation of sendmail+IDA plus a binary of the mailx user agent. All file paths below this directory reflect the location where the files should be installed, so it's safe to work up a tar command to move 'em over:
# cd sendmail5.65b+IDA1.5+mailx5.3b # tar cf - . | (cd /; tar xvvpoof -)