If your mail is being blocked by a mailbox provider:
1. Review your bounce logs.
- 550 SC-003: Mail rejected by Outlook.com for policy reasons. Your IP address appears to be an open proxy or relay. If you are not an email or network administrator, please contact your mailbox provider for help.
- AOL 421 DYN: T1 block. This is a temporary block caused by high complaints or low IP reputation.
- A poor IP or domain sending reputation, which may reflect a high complaint rate, high number of unknown users, sending to spam traps, and low engagement
- Access denied (being in the queue too long)
- An IP address or domain listed on a blocklist
- Spam-like characteristics (search for spam or abuse in your bounce logs)
- Too many connections opened to the receiving server and sending too many emails in a short period of time (connection and throughput)
- A new IP address or domain
- Sending to a suppression list
2. Assess and troubleshoot the situation.
- Consider the following questions:
- Are all mailings being blocked, or is it a partial block?
- Is the block across all mailbox providers or just one?
- Is the block widespread or is it isolated to a specific day, date, time, IP address, domain, or mail stream?
- When did the block start? Is it getting worse or better?
- Is the block constant or sporadic?
- Ensure that email was not accidentally sent to a suppression file.
- Review your complaint handling process and ensure all complainers no longer receive email.
- Ensure your bounce handling process is working correctly and that unknown users are removed after one to two bounces.
- Review your list acquisition methods. Did you acquire or purchase new addresses recently from a third party?
- Review your list hygiene methods to ensure you are sending to people that want to receive your email and engage with you frequently based on your business
3. Review all other data sources about the mailings to identify negative reputation metrics relating to the bounce codes, time frame, IP addresses, mail streams, and domains as described in steps 1 and 2 above.
- Your email and engagement data
- Validity Certification data
- Microsoft Smart Network Data Services (SNDS) data if the policy block is from Outlook
4. Fix the root cause of the block.