An SMTP service is must in order to resolve the deliverability issues, limitations, you face while sending emails through your WordPress website.
With an SMTP service, seamless email deliverability directly from your WordPress website becomes a reality.
Mailgun WP SMTP plugin connects Mailgun to your WordPress website and bypasses the default wp_mail() PHP function in WordPress and sends your email via Mailgun SMTP service.
Now the Mailgun SMTP service will take charge of your emails, and make sure your emails will be delivered at maximum.
SMTP interacts with the email server and validates your emails as per the server protocols. You might have several question peeking in your mind. Here are several good articles as reference and they may help you otherwise -
SMTP (Simple Mail Transfer Protocol) in simple terms, is a proper infrastructure for sending emails.
SMTP helps deliver your emails in a more authentic manner to bring about a better delivery rate. Otherwise several numerous reasons may cause your emails to fall in your user's 'spam emails' folder, or remain undelivered.
Maximum of your clients use email service either of Gmail, Yahoo, Outlook, etc. They have insanely enormous email transfers to manage, and they have strict rules to prevent spam emails from reaching their users.
With an SMTP service, you satisfy their protocol, and get your emails delivered right in your client's inbox. It handles all of the heavy lifting to bring about a better of email delivery rate.
So you know the importance of using a dedicated SMTP service. Now, this Mailgun SMTP WP plugin lets you add more facility to your WordPress website, if you use Mailgun SMTP service. However, there are serveral
smtp service providers also available.
Below are some more sources of information that you might need in this regard.