Three causes, ranked by frequency.
Wrong billing contact
You're emailing the project contact instead of accounts payable. Open the customer record → Contacts tab → mark the right person as billing_contact = true.
SMTP reputation drop
If one account in the pool hits a spam complaint, deliverability drops for everything routed through it. Settings → Email → SMTP pool → check health flags. Pause unhealthy accounts; the pool routes around them.
Domain auth misconfigured
SPF/DKIM/DMARC on your sender domain. Settings → Email → Diagnostics will flag the specific record missing. Fix at your DNS provider; allow 24h to propagate.