Step 1: Install WP Mail Logging. Always the first step.
Step 2: Place a test order (use a 100% coupon).
Step 3: Check the mail log (Tools → WP Mail Log).
Email appears in log → sent but not received. Deliverability problem. Check spam folders. Verify FROM uses your domain (not @gmail.com). Contact hosting about SPF/DKIM/DMARC. Consider an SMTP plugin.
Email not in log → never generated. Site-level problem. Deactivate SMTP plugin and test. If emails generate without it, the SMTP config needs fixing. If still nothing, run conflict testing (3.1).
For Gmail/Yahoo specifically: Since 2024, both require proper authentication. Without SPF, DKIM, and DMARC, emails may be silently dropped — not even spam folder. This is the #1 “emails suddenly stopped” cause.