Email Providers

While SMTP is a standardized protocol, different email providers implement it in slightly different ways. Also, feedback loops (FBLs) are not standardized, and each email provider has its own way of reporting spam complaints. This document outlines notable differences among popular email providers and how Hyvor Relay handles them.

• Feedback Loops (FBLs)
• SMTP Responses

Feedback Loops (FBLs)

When a recipient marks an email as spam, the email provider may send a complaint to the sender via a feedback loop (FBL). When hosting Hyvor Relay, you may sign up for these FBLs to monitor spam complaints and route FBL reports abuse@ and fbl@ emails of your instance domain. Hyvor Relay's incoming SMTP server will process these reports and automatically add the recipients to the suppression list.

Yahoo FBL

SMTP Responses

Below we have listed some of the enhanced SMTP codes used by popular email providers.

Recipient error codes indicate issues with the recipient's email address. If Hyvor Relay encounters these codes, it will add the recipient to the suppression list.

Infrastructure error codes indicate issues with Hyvor Relay's sending infrastructure (e.g., an IP address being blocked). Those codes are serious and may impact email deliverability for multiple recipients. A remedy must be applied by the administrator of the Hyvor Relay instance. Responses are logged and can be viewed in the Sudo → Debug → Infrastructure Bounces section. There is also a sudo health check to alert if infrastructure bounces are detected.

Hyvor Relay does not suppress recipients if an enhanced SMTP code is not set in the response (e.g., Yahoo returns a generic 550 without an enhanced code).