specification: API Commons Rate Limits
specificationVersion: '0.1'
schema: https://raw.githubusercontent.com/api-evangelist/interface-research/main/schema/api-commons.yml#/$defs/RateLimits
provider: Microsoft Outlook
providerId: microsoft-outlook
created: '2026-05-04'
# Provenance stamped 2026-08-11: this artifact was written by the API Evangelist
# bulk sweep dated 2026-05-04, not harvested from the provider. See roadmap#35.
method: generated
modified: '2026-05-05'
reconciled: true
tags:
  - Rate Limiting
  - Email
  - Calendar
  - Outlook
  - Microsoft Graph
  - Microsoft
description: Outlook programmatic access is via Microsoft Graph mail / calendar / contacts endpoints (preferred),
  Exchange Web Services (legacy), and the deprecated Outlook REST v2.0. Throttling is enforced per app/mailbox
  and at the Microsoft Graph global ceiling. SMTP submission and recipient caps apply to Exchange Online
  mailboxes regardless of API used. Returns 429 with Retry-After.
sources:
  - https://learn.microsoft.com/en-us/graph/throttling
  - https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/exchange-web-services/ews-throttling-in-exchange-online
  - https://learn.microsoft.com/en-us/exchange/troubleshoot/send-emails/smtp-submission-limits
headers:
  retryAfter: Retry-After
  requestId: request-id
  throttleScope: x-ms-throttle-scope
  throttleLimit: x-ms-throttle-limit-percentage
responseCodes:
  throttled: 429
  serviceUnavailable: 503
limits:
  - name: Microsoft Graph global ceiling
    scope: application
    metric: requests_per_10_seconds
    limit: 130000
    timeFrame: second
  - name: EWS — concurrent connections per app/mailbox
    scope: app/mailbox
    metric: concurrent_connections
    limit: 27
    notes: Hard cap of 27 concurrent EWS connections per (application, mailbox) tuple
  - name: EWS — total per mailbox
    scope: mailbox
    metric: concurrent_connections
    limit: 27
  - name: SMTP submission per mailbox
    scope: mailbox
    metric: messages_per_minute
    limit: 30
    timeFrame: minute
  - name: SMTP recipients per mailbox
    scope: mailbox
    metric: recipients_per_day
    limit: 10000
    timeFrame: day
  - name: Outlook (mail / calendar / contacts) on Microsoft Graph
    scope: app/mailbox
    metric: varies
    limit: per-mailbox throttling — fall back to Retry-After. No fixed numeric per-second value published.
  - name: Identity & Access (read) per app/tenant (used for user lookups)
    scope: app/tenant
    metric: resource_units_per_10_seconds
    limit: 3500–8000 RU per 10s based on tenant size
    timeFrame: second
policies:
  - name: Backoff Strategy
    description: Honour Retry-After on 429; the Microsoft Graph SDKs implement this automatically.
  - name: Use Microsoft Graph over EWS / Outlook REST
    description: EWS is on a deprecation path and Outlook REST v2.0 is deprecated for work / school accounts;
      target Microsoft Graph for new development.
  - name: Use change notifications
    description: Subscribe to mail and calendar change notifications instead of polling.
  - name: SMTP relay for high volume
    description: For application-generated mail above mailbox SMTP caps, use a connector or a third-party
      relay; the 30 msg/min and 10K recipients/day limits are hard caps.
  - name: Use $batch
    description: Combine related calls in $batch envelopes; each is throttled individually but network
      overhead is reduced.
maintainers:
  - FN: Kin Lane
    email: kin@apievangelist.com