Glossary

SMTP relay

SMTP relay

SMTP relay is the process of transferring email messages between mail servers using SMTP. For example, when you send an email from your app to Mailgun’s SMTP endpoint, Mailgun acts as the relay, passing the message along to the recipient’s server. Relay services can be: 

  • Open relays: Publicly accessible and often abused (not recommended) 
  • Closed relays: Only allow authenticated, internal, or approved traffic 

Most ESPs (like Mailgun) operate closed SMTP relays to protect against abuse. 

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