Make it possible to only use SMTP for some sites in a multisite (#111)

* make it possible to only use SMTP for some sites in a multisite

* change setting from string to array

README.md

@@ -205,6 +205,7 @@ configuration.
 `BM_WP_SMTP_PASSWORD` - Define the SMTP e-mail account password to send through.
 `BM_WP_SMTP_PORT` - Set which port the connection should be made through. Should be set as an integer. Defaults to `587`.
 `BM_WP_SMTP_SECURITY` - Set the sending security for the SMTP server. Defaults to `tls`.
+`BM_WP_SMTP_SITE_IDS` - Set side ids in an array to only use SMTP for some sites in a multisite. Defaults to work for all sites.
 
 `BM_WP_POSTAL_DOMAIN` - Optionally customize the domain for the Postal install. Defaults to `https://postal.oderland.com`
 `BM_WP_POSTAL_API_KEY` - Add the API key for the Postal service. Without it, Postal is not active.

src/Modules/Mail.php

@@ -22,6 +22,10 @@ public static function warn_improper_smtp_configuration(): void {
 			return;
 		}
 
+        if( ! self::should_send_for_this_site() ){
+            return;
+        }
+
 		if ( self::are_all_configs_set() ) {
 			return;
 		}
@@ -39,6 +43,7 @@ public static function warn_improper_smtp_configuration(): void {
 
 	public static function send_mail_via_smtp( PHPMailer $mailer ): void {
 
+        if( ! )
 		if ( ! self::should_send_via_smtp() ) {
 			return;
 		}
@@ -56,6 +61,22 @@ public static function send_mail_via_smtp( PHPMailer $mailer ): void {
 		$mailer->isSMTP();
 	}
 
+    protected static function should_send_for_this_site(): bool {
+        if( ! is_multisite() ){
+            return true;
+        }
+
+        if( ! defined( 'BM_WP_SMTP_SITE_IDS' ) ){
+            return true;
+        }
+
+        if( is_array( BM_WP_SMTP_SITE_IDS ) && in_array( get_current_blog_id(), BM_WP_SMTP_SITE_IDS ) ){
+            return true;
+        }
+
+        return false;
+    }
+
 	protected static function should_send_via_smtp(): bool {
 		if ( 'production' !== wp_get_environment_type() ) {
 			return self::should_send_smtp_outside_production();