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TLS and SSL
---------------------------------------------------------------------
TLS and SSL are almost same. Over the years vulnerabilities have been
and continue to be discovered in the deprecated SSL protocols, so
use TLS.
---------------------------------------------------------------------
$mail->SMTPSecure = 'tls';
---------------------------------------------------------------------

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About STARTTLS ( doesnot meant TLS only! )
---------------------------------------------------------------------
STARTTLS is a way to take an existing insecure connection and upgrade
it to a secure connection using SSL/TLS. Note that despite having TLS
in the name, STARTTLS doesn't mean you have to use TLS, you can use
SSL.

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Transport
---------------------------------------------------------------------
a. No need to do anything to use PHP's mail() function

b. Execute isSendmail() to use sendmail libraray
	$mail->isSendmail();

c. Execute isSMTP() to use SMTP protocal
	$mail->isSMTP();

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Authentication and Encryption
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To use authentication, execute $mail->SMTPAuth = true;
To use encryption, execute $mail->SMTPSecure = 'tls';

"SMTPSecure" decides which security to use, TLS or SSL.
However, it doesn't decide whether or not security is used.
It looks like automatic. If the server gives an option of SSL/TLS
connection, it uses one.

-----------------------------------
CLIENT -> SERVER: STARTTLS
-----------------------------------

Then the server returns

-----------------------------------
SERVER -> CLIENT: 220 TLS go ahead
-----------------------------------

Now the client can sent data in an encrypted mode.

If the server doesn't give an option of security,
then the client just transmits data without encryption.
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//-- No Authentication ( encryption is auto )
$mail->isSMTP(); // Use SMTP
$mail->Host = "mail.phpkobo.com";
$mail->Port = 25;
$mail->SMTPAuth = false;

//-- Authentication ( encryption is auto )
$mail->isSMTP(); // Use SMTP
$mail->Host = "mail.devpacc.com";
$mail->Port = 25;
$mail->SMTPAuth = true;
$mail->Username = "info+devpacc.com";
$mail->Password = ".......";

//-- Gmail (Authentication)
$mail->isSMTP(); // Use SMTP
$mail->Host = 'smtp.gmail.com';
$mail->Port = 587;
$mail->SMTPSecure = 'tls';
$mail->SMTPAuth = true;
$mail->Username = "phpkobo@gmail.com";
$mail->Password = ".......";

=====================================================================