Unlike consumer ISPs, Gmail uses and post-delivery adjustments . An email delivered to the Inbox today can be retroactively moved to Spam or blocked tomorrow if user behavior signals change.
If recipients frequently mark your emails as spam, your "sender score" drops, making it more likely that Gmail will block your future messages. How to Fix Outgoing Blocks blocked emails gmail
| Error Code | Meaning | Action | |------------|---------|--------| | 550 5.7.1 | General block (policy) | Check DMARC, IP reputation | | 550 5.7.26 | SPF/DKIM failure | Fix DNS records | | 550 5.7.28 | Domain in Google’s blocklist | Request removal via postmaster.google.com | | 421 4.7.0 | IP rate-limited | Slow down sending | | 450 4.2.1 | User over quota | Remove address from list | | 550 5.7.23 | Message contains malware | Scan attachments | How to Fix Outgoing Blocks | Error Code