Deprecated watchdog function needs to be replaced with logger

@jorgik, there are 2 minor issues with your patch:

1) this line
\Drupal::logger('smtp')->error( 'There is no submitted from address.');
contains an extra space

2) In this line
drupal_set_message(t('The submitted from address @from is not valid.', array('@from' => $from)), 'error');
I believe no need to remove parenthesis around @from. Or at least we should change it to %from to emphasise core element of the message.

Thank you, #9 looks nice!

It looks nice but maybe we can inject the logger in place of the procedural way. I think that mail system are very relevant in all sites and it's useful make it configurable in the class constructor.

Review this link https://www.drupal.org/node/2595985 for checking it.

(But anyway, the patch apply correctly and is correct).

OK, I changed patch, added logger in constructor, needs review

+++ b/src/Plugin/Mail/SMTPMailSystem.php
@@ -10,27 +10,45 @@ namespace Drupal\smtp\Plugin\Mail;
-use Drupal\smtp\PHPMailer\PHPMailer;
...
+use PHPMailer;

@@ -74,7 +92,7 @@ class SMTPMailSystem implements MailInterface {
-    // Create a new PHPMailer object - autoloaded from registry.
+    // Create a new PHPMailer object - autoloaded from composer.

I think that you has mixed two different issues in the patch ;)

(The other part seems nice, please fix it and I will review after it)

Sorry, fixed

Tested on the last update from 8.x-1.x branch

+++ b/src/Plugin/Mail/SMTPMailSystem.php
@@ -90,10 +104,10 @@ class SMTPMailSystem implements MailInterface {

-        $from = $this->smtpConfig->get('smtp_from');
-        $headers['Sender'] = $from;
-        $headers['Return-Path'] = $from;
-        $headers['Reply-To'] = $from;

+      $from = $this->smtpConfig->get('smtp_from');
+      $headers['Sender'] = $from;
+      $headers['Return-Path'] = $from;
+      $headers['Reply-To'] = $from;

@@ -159,25 +173,25 @@ class SMTPMailSystem implements MailInterface {
-            break;
+              break;
...
-            break;
+              break;
...
-            break;
+              break;

This isn't related with the current issue, are only indentation changes. Is good idea resolve one task in each issue. If you want to refactor the code to use the coding standard, please open another issue.

Ok, fixed in new patch

I tried to send an email, and watchdog throw me this message:

Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "smtp.logger.channel". en Drupal\Component\DependencyInjection\Container->get() (linea 157 de /home/rrqun/www/core/lib/Drupal/Component/DependencyInjection/Container.php).

Can you test it again, please?

Sorry, tested, with correct of name service http://joxi.net/a2X3vYGhyP5NYm.png

Patch work perfect, messages save in watchdog correctly. I think that all is ok.

But please, in order to ease the review, make an interdiff in this kind of patchs with little changes between versions.
https://www.drupal.org/documentation/git/interdiff