When emails don't bounce or go to spam but aren't delivered, Proofpoint is likely quarantining them. The problem might stem from bad content, unengaged recipients, or IP reputation. Resolutions involve contacting Proofpoint support, checking Cloudmark and SORBS, utilizing Proofpoint's false positive/negative reporting system, and allowing users to manage their quarantine. Email list maintenance, proper authentication (SPF, DKIM, DMARC), dedicated IPs and warming them up, and proactive sender reputation monitoring (Google Postmaster Tools, Microsoft SNDS), alongside email testing, also improve deliverability. Subscriber advocacy, involving email admins for whitelisting, is key.