I have used pear mail a number of times.
the error message was pretty clear:
open_basedir restriction in effect. File(/usr/bin/pear/Mail.php) is not within the allowed path(s): (.:/php/includes:/usr/local/pem/vhosts/126152)
it is saying that the allowed paths for the open_basedir restriction are:
and that the file for pear mail:
is being denied access because it is not in one of those 2 allowed paths.
the host would need to add the path:
to the open_basedir restriction for pear to work.