The 550 High Probability of Spam SMTP (Simple Mail Transfer Protocol) error means your email was rejected because the receiving server flagged it as likely spam.
How it works
When an email fails checks like SPF/DKIM, hits spam filters, or comes from a suspicious IP or domain, the recipient’s server might block it outright.
The “550” code is a generic permanent failure; the “high probability of spam” part tells you why.
Why it matters
Your sender reputation might be damaged, or something in your message triggered a filter. Repeated errors like this can tank deliverability.
Tips
- Check your SPF/DKIM/DMARC setup
- Avoid spammy subject lines or too many links
- Use warmup tools like Warmup Inbox to rebuild your reputation