PHP PEAR was not installed on my 1&1 VPS (CentOS 7) at all, so the first step was to to install it:
yum install php-pear
Next, install PEAR Mail and Net_SMTP packages:
pear install mail
pear install Net_SMTP
PEAR was installed to /usr/share/pear path which was automatically added (or maybe was there before...) to the default include_path in Plesk PHP settings.
Still, when PHP was trying to use PEAR Mail, following error was logged:
Warning: require_once(): open_basedir restriction in effect. File (/usr/share/pear/Mail.php) is not within the allowed path(s)...
As the error suggests, Mail.php package is found but PHP's open_basedir was blocking access to it. To resolve this I had to add /usr/share/pear path to open_basedir in PHP settings within Plesk:
The full open_basedir string now was: {WEBSPACEROOT}{/}{:}{TMP}{/}:/usr/share/pear
February 2017
1&1 VPS Web Server
Plesk Onyx 17
CentOS 7
PHP
PEAR Mail
