Return values are a plugin's way of communicating back to the qpsmtpd core, and an integral part of the Plug-in API

Some of the most common return values are

The OK return value implies that the plugin has performed some sort of checking on this connection or transaction, and that the remaining plugins should not be called for the current hook. This is not very clear ¹⁾

The DENY return value tells qpsmtpd to end this transaction with a 5xx_error_code.

The DENY_DISCONNECT return value tells qpsmtpd to end this transaction with a 5xx_error_code and to end the connection by “hanging up” on the connecting client.

The DENYSOFT return value tells qpsmtpd to end this transaction with a 4xx_error_code, i.e. a temporary error.

The DENYSOFT_DISCONNECT return value tells qpsmtpd to end this transaction with a 4xx_error_code, i.e. a temporary error, and to end the connection by “hanging up” on the connecting client.

The DECLINED return code is used to indicate that the plugin does not wish to affect reception of this message, or that the plugin has failed internally. It will imply that the next plugin is called, so many plugins will use DECLINED and not OK to signal that this plugin thinks the message is legitimate. This is not very clear either ²⁾