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The installation of PHPMailer via manual process may be unsuccessful when it comes to smtpSend because of an undefined constant due to several reasons such as:

1. Incorrect path to the PHPMailer library: The path to the PHPMailer library may be incorrect, leading to an undefined constant error.

2. Incorrect configuration settings: The configuration settings such as SMTP server, username, password, and port may be incorrect, leading to an undefined constant error.

3. Outdated version of PHP: PHPMailer requires a minimum version of PHP 5.5.9, and if the installed version of PHP is outdated, it may lead to an undefined constant error.

4. Missing dependency libraries: PHPMailer depends on several third-party libraries such as Open SSL, which may be missing in the system, leading to an undefined constant error.

5. Typographical errors: Typographical errors may also cause an undefined constant error if the code is not correctly written.