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Fix Outlook Python Automation Mailbox Size Limit Error

Maira Mehtab Sep 22, 2026 869 views

Question details

A Python script using Outlook automation fails to send email messages due to a mailbox-size limit error, whereas manually created emails send successfully without triggering this error.

Product: Microsoft Outlook
Device & OS: not provided
Scenario: Sending automated emails with a Python script via the Outlook COM interface.
Observed behavior: The automated script fails to send messages and returns a mailbox size limit error, while manual email sending works normally under the same account.

Before you start

Before modifying your Python code, log into Outlook on the web (OWA) and verify your actual mailbox usage to confirm your account has not genuinely reached its storage quota.

Solution 1Recommended

Diagnose Generated Message Size and Content

Determine if the Python automation script is unintentionally creating bloated content by saving the email as a draft and comparing its size to a manual message.

Python automation scripts can sometimes embed files incorrectly or encode attachments in a way that exponentially increases the overall payload size, triggering client-side limitations.

Collect Complete Error Details

Review the Python console traceback to extract the complete error text. Identify whether the error is a local client rejection or a server-side Exchange limit.

Modify Python Script to Save as Draft

Change your script's output method from `.Send()` to `.Save()`. This places the generated email into your Outlook Drafts folder instead of attempting to send it immediately.

Inspect the Draft Message Size

Open the Outlook desktop client, navigate to the Drafts folder, and check the file size of the generated message. Compare this to the size of a manually drafted message containing the exact same text and attachments.

Check for Duplicated or Oversized Content

Open the draft email and verify that the Python script has not attached the same file multiple times or embedded large Base64 images directly into the HTML body, which inflates the size.

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Frequently Asked Questions

Why does my Python script trigger a size error when manual emails do not?

Scripts often handle attachments differently, such as embedding large files directly into the email body via Base64 encoding. This can inflate the total message size by over 30%, triggering the server's message or mailbox limit unexpectedly.

How do I check my current mailbox size limit in Outlook?

In the Outlook desktop client, click on the File tab. Under the Info section, look for 'Mailbox Settings' to see a visual storage bar indicating how much space you have used versus your total capacity.

Does Outlook on the web have the same automation limits as the desktop client?

Outlook on the web processes messages directly on the Exchange server. If you receive an error locally via COM automation, testing the same attachments in Outlook on the web can help determine if the issue is a local client limit or a strict server-side restriction.

Outlook & Email Problems