mssendmail

Microsoft Graph-based sendmail replacement with a persistent queue for Linux

mssendmail provides a drop-in sendmail interface for sending emails via the Microsoft Graph API. It stores emails in a local queue and processes them asynchronously via systemd-managed background worker.

Features

• Drop-in compatible with basic sendmail calls (e.g. for Mastodon)
• Persistent queue (handles temporary outages)
• Sends via Microsoft Entra ID + Graph API
• Environment-based config via /etc/mssendmail/.env
• File-based logging

Prerequisites

1. An Shared Mailbox in Exchange Online
2. An App Registration
3. A Client Secret
4. Assign API Permissions
   • Add Mail.Send as application permission
5. Restrict App Access to specific Mailboxes New-ApplicationAccessPolicy -AppId "" -PolicyScopeGroupId "" -AccessRight RestrictAccess

Installation

• Install uv
• Clone this repo
  • uv pip install .
  • Registers the commands mssendmail and mssendmail-worker
• Create user mssendmail sudo useradd --system --home /var/lib/mssendmail --shell /usr/sbin/nologin mssendmail
• Create folder sudo mkdir -p /var/lib/mssendmail/queue /var/log/mssendmail /etc/mssendmail sudo chown -R mssendmail:mssendmail /var/lib/mssendmail /var/log/mssendmail sudo chmod 750 /var/lib/mssendmail /var/log/mssendmail sudo touch /etc/mssendmail/.env sudo chown root:mssendmail /etc/mssendmail/.env sudo chmod 640 /etc/mssendmail/.env
• Configure
• Create a user
• Create required directories
• Add the systemd service for the mssendmail-worker
  • cp util/mssendmail-queue-worker.service /etc/systemd/system
  • systemctl daemon-reexec
  • systemctl enable --now mssendmail-queue-worker.service

Configuration

File: /etc/mssendmail/.env

TENANT_ID=your-tenant-id CLIENT_ID=your-app-client-id CLIENT_SECRET=your-app-secret SENDER=noreply@yourdomain.com

QUEUE_DIR=/var/lib/mssendmail/queue LOG_DIR=/var/log/mssendmail

Manual Usage

Queue a mail: cat mail.eml | mssendmail

Manually process queue: mssendmail-worker

In the uv

cat mail > uv run mssendmail/queue_writer.py

Missing features

• Retry mechanism is missing backoff
• Logrotation
• Packaging