It's very unlikely that the version of WordPress you use affects your email delivery. WP Mail SMTP is as effective as Postman when properly configured, for non-OAuth2 servers anyway.
If you have a problem receiving the email sent by your system, it is likely that your own email server is periodically rejecting emails for an SPF or Spam violation. Since each case is unique it is impossible to provide a "this is how to fix your email" solution that fits everyone. But I believe every case is ultimately fixable.
Postman's email log records the session transcript of outgoing emails. That transcript is the key to resolving your email issues.