Instead, configure your web server on the Operating System level with Postfix. This is better way since Postfix will handle the rest. Also, provided if there is latency between the web server and the email provider, it will not affect your WordPress performance.
Bad news
Some WordPress hosting provider force their customer to use WordPress SMTP plugin instead of configuring the hosting environment with PHP mail function. This will result to latency when WordPress deals with email sending activity (reset password, payment received, etc).
The issue
WordPress didn’t have background processing system out-of-the-box like most modern web framework has (Ruby on Rails – Active Job, Laravel – Queues, etc). So, everything will and must be done on Apache/Nginx request or at WordPress Cron Jobs (at best).
PHP mail are used by WordPress to send an email where if the web server are not configured with Mail Transfer Agent (MTA) like Postfix, the email will never be sent. So, in case of configuring MTA is not an option, the only option you have is to use WordPress email plugin.
Consider this
Since WordPress didn’t have background processing system, email sending will be done in real-time. When the email server is slow or far or having high latency for whatever reason, this will affect your WordPress site performance as it will have to wait for the email sending request to be completed.
What should you do?
If you are looking for hosting provider, search for hosting provider that offer hosting package that comes with built-in email features. You should create a support ticket to confirm if the specific hosting plan requires additional plugin for an email feature to be enabled on your WordPress site. If yes, then, you should think twice.
Provided you are running the server instance on your own, you should consider configuring Postfix (or any Mail Transfer Agent). Let the operating system handle the most part of email and WordPress handle your blog. That is the best configuration.
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