I'm having the exact same problem now with installing Net_SMTP on wampstack 7.1.10-0
Here is the output:
C:\Bitnami\wampstack-7.1.10-0-HBFINAL>pear install pear/mail
Did not download optional dependencies: pear/Net_SMTP, use --alldeps to download automatically
pear/Mail can optionally use package "pear/Net_SMTP" (version >= 1.4.1)
downloading Mail-1.4.1.tgz ...
Starting to download Mail-1.4.1.tgz (21,756 bytes)
........done: 21,756 bytes
could not extract the package.xml file from "C:\Bitnami\wampstack-7.1.10-0-HBFINAL\php\pear\download\Mail-1.4.1.tgz"
Download of "pear/mail" succeeded, but it is not a valid package archive
Error: cannot download "pear/Mail"
This happens with any package I try to install.
any ideas, and is anyone else having this problem?