Some folks set up an SMTP plugin to use their existing email provider (e.g. Gmail, Outlook.com, Yahoo, etc) to send their WordPress emails but then find out the hard way (i.e. emails not getting delivered) that there’s a daily hard limit on the number of emails they can send. Sending WordPress emails through SMTP is simply not worth the risk.
Are your WordPress site emails not being delivered? That’s pretty common. Over 20,000 sites trust WP Offload SES Lite to send their site email.
WordPress’ default email sending functions just don’t cut it these days. You absolutely need to set up something more.