MIDDLEWARE.md

Middleware support

Microsoft Graph SMTP Relay supports a simple EventBus implementation allowing to subscribe to different event names.
Also, the MIDDLEWARE_DIR environments variable defines a location to load additional python modules as middleware (default app/middleware/*.py)

Example of a middleware implementation

Example middleware using the skip_send event to skip email submission

# app/middleware/skip_sendmail.py
from event_bus import event_bus_instance
from handlers.microsoft_graph import MicrosoftGraphHandler

class Middleware:
    def __init__(self, msGraphHandler: MicrosoftGraphHandler):
        # ...
        event_bus_instance.subscribe('skip_send', self.skipSendmail)

    def skip_sendmail(self):
        # return True to skip email submission
        return True

Another example on how to modify email subject

class Middleware:
    """
    Middleware to demonstrate EmailMessage modifications by adding a prefix to the "Subject" header
    """

    def __init__(self, msGraphHandler: MicrosoftGraphHandler):
        self.app = msGraphHandler.app

        event_bus_instance.subscribe('before_send',self.modify_mail)

    def modify_mail(self, mail_message: EmailMessage):
        subject = mail_message["Subject"]
        logging.info(f"ReplaceSubject -> Adding prefix to subject")
        mail_message.replace_header('Subject', '[ReplaceSubject middleware] ' + subject)

Another example to overwrite the Reply-To header

from event_bus import event_bus_instance
from handlers.microsoft_graph import MicrosoftGraphHandler

class Middleware:
    """
    Middleware to overwrite the Reply-To Header
    """
    def __init__(self, msGraphHandler: MicrosoftGraphHandler):
        self.app = msGraphHandler.app
        event_bus_instance.subscribe('sender', self.overwrite_reply_to)

    async def overwrite_reply_to(self, sender_mail: str | None, reply_to: list):
        # Overwrite reply to address
        reply_to.clear()
        reply_to.append({"emailAddress": {"address": "some@email.tld"}})
        
        return True

Event subscriptions and EventBus

Below class graph displays methods in the EventBus

Example

# subscribe to "sender" event and call my_sender_func when published
from event_bus import event_bus_instance
event_bus_instance.subscribe('sender', self.my_sender_func)

List of Event subscriptions

Event name	Arguments	Description
before_auth	auth_data	before authorization
after_auth	auth_data	after authorization
before_send	email_message	before email submission
sender	mail_from	When sender is known
recipients	to, cc, bcc	When recipients are known
skip_send		Used to skip mail submission (dependent on return value)
after_send		after email submission