I couldn’t install the Mail PEAR package because the pear version shipped with the current CentOS/RHEL is 1.4.9, whereas the required version is 1.5.6 or above. The following is the error that you may see.

# pear install Mail
WARNING: channel "pear.php.net" has updated its protocols, use "channel-update pear.php.net" to update
Did not download optional dependencies: pear/Net_SMTP, use --alldeps to download automatically
pear/Mail requires PEAR Installer (version >= 1.5.6), installed version is 1.4.9
pear/Mail can optionally use package "pear/Net_SMTP" (version >= 1.4.1)
No valid packages found
install failed

There are three problems above:

1. WARNING: channel “pear.php.net” has updated its protocols, use “channel-update pear.php.net” to update
2. Did not download optional dependencies: pear/Net_SMTP, use –alldeps to download automatically
3. Main problem: pear/Mail requires PEAR Installer (version >= 1.5.6), installed version is 1.4.9

To fix problem #1 (update channel pear.php.net):

# pear channel-update pear.php.net
Retrieving channel.xml from remote server
Update of Channel "pear.php.net" succeeded

To fix problem #2 (Net_SMTP is not installed), run:

# pear install --alldeps Mail 
pear/Mail requires PEAR Installer (version >= 1.5.6), installed version is 1.4.9
downloading Net_SMTP-1.4.4.tgz ...
Starting to download Net_SMTP-1.4.4.tgz (12,264 bytes)
.....done: 12,264 bytes
downloading Net_Socket-1.0.10.tgz ...
Starting to download Net_Socket-1.0.10.tgz (5,429 bytes)
...done: 5,429 bytes
downloading Auth_SASL-1.0.4.tgz ...
Starting to download Auth_SASL-1.0.4.tgz (5,795 bytes)
...done: 5,795 bytes
install ok: channel://pear.php.net/Auth_SASL-1.0.4
install ok: channel://pear.php.net/Net_Socket-1.0.10
install ok: channel://pear.php.net/Net_SMTP-1.4.4

To fix problem #3 (old pear version), first upgrade pear:

# pear upgrade pear
pear/PEAR dependency package "pear/Structures_Graph" downloaded version 1.0.4 is not the recommended version 1.0.3, but may be compatible, use --force to install
pear/PEAR dependency package "pear/Console_Getopt" downloaded version 1.3.0 is not the recommended version 1.2.3, but may be compatible, use --force to install
pear/Archive_Tar requires PEAR Installer (version >= 1.5.4), installed version is 1.4.9
pear/Console_Getopt requires PEAR Installer (version >= 1.9.1), installed version is 1.4.9
downloading Structures_Graph-1.0.4.tgz ...
Starting to download Structures_Graph-1.0.4.tgz (30,318 bytes)
.........done: 30,318 bytes
downloading XML_Util-1.2.1.tgz ...
Starting to download XML_Util-1.2.1.tgz (17,729 bytes)
...done: 17,729 bytes
upgrade ok: channel://pear.php.net/XML_Util-1.2.1
upgrade ok: channel://pear.php.net/Structures_Graph-1.0.4

But this still doesn’t update base pear, so run:

# pear upgrade --force pear
warning: pear/PEAR dependency package "pear/Console_Getopt" downloaded version 1.3.0 is not the recommended version 1.2.3
warning: pear/Archive_Tar requires PEAR Installer (version >= 1.5.4), installed version is 1.4.9
warning: pear/Console_Getopt requires PEAR Installer (version >= 1.9.1), installed version is 1.4.9
downloading PEAR-1.9.1.tgz ...
Starting to download PEAR-1.9.1.tgz (293,587 bytes)
.............................................................done: 293,587 bytes
downloading Archive_Tar-1.3.7.tgz ...
Starting to download Archive_Tar-1.3.7.tgz (17,610 bytes)
...done: 17,610 bytes
downloading Console_Getopt-1.3.0.tgz ...
Starting to download Console_Getopt-1.3.0.tgz (4,408 bytes)
...done: 4,408 bytes
upgrade ok: channel://pear.php.net/Console_Getopt-1.3.0
upgrade ok: channel://pear.php.net/Archive_Tar-1.3.7
upgrade ok: channel://pear.php.net/PEAR-1.9.1
PEAR: Optional feature webinstaller available (PEAR's web-based installer)
PEAR: Optional feature gtkinstaller available (PEAR's PHP-GTK-based installer)
PEAR: Optional feature gtk2installer available (PEAR's PHP-GTK2-based installer)
To install use "pear install pear/PEAR#featurename"

Now we can install pear’s Mail:

# pear install --alldeps Mail
downloading Mail-1.2.0.tgz ...
Starting to download Mail-1.2.0.tgz (23,214 bytes)
........done: 23,214 bytes
install ok: channel://pear.php.net/Mail-1.2.0

You might also need Mail_mime:

# pear install Mail_mime
downloading Mail_Mime-1.8.1.tgz ...
Starting to download Mail_Mime-1.8.1.tgz (31,530 bytes)
.........done: 31,530 bytes
install ok: channel://pear.php.net/Mail_Mime-1.8.1