Greylisting

Email greylisting is a spam-filtering technique used by some receiving mail servers. When an email is received from an unfamiliar sender (i.e., a new IP/domain combination), the server temporarily rejects it with a 4xx error, signaling the sender to try again later. 

Legitimate senders will retry delivery after a short delay, at which point the receiving server allows the message through. Many spammers and low-quality bots don’t retry, so greylisting acts as a passive filter for low-effort spam. 

Key points: 

• Greylisting is used by recipients, not senders. It’s a filtering mechanism, not something Mailgun customers can apply. 
• It can cause slight delivery delays on first-time sends from a new IP or domain. 
• Once a sender is “known,” future messages usually bypass the delay. 
• Mailgun automatically retries deferred messages as part of its standard delivery process, so most customers don’t need to take any action.