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Fix Outlook Mailbox Size Limit Error When Sending Email with Python
Question details
Users encounter a mailbox size limit error when attempting to send emails programmatically using a Python script and win32com, despite the mailbox having available space.
- Product
- Microsoft Outlook, Python (win32com)
- Device & OS
- not provided
- Scenario
- Automating email sending via Python scripts using the Outlook desktop application.
- Observed behavior
- A mailbox size limit error is returned when executing the Python script, whereas manually composing and sending the same email works normally.
Verify that your Outlook client is successfully connected to the Exchange server and ensure that your Python script's environment has the necessary permissions to access the local COM components.
Diagnose Account Constraints and Attachment Sizes
Isolate the issue by checking web mail functionality and verifying if attachments are triggering false size limit errors via win32com.
Sometimes, the COM interface handles attachments and encoding differently than the manual Outlook GUI, resulting in a payload that exceeds programmatic limits and triggers a generic mailbox size error.
Log into Outlook on the web at outlook.office.com to verify your actual mailbox quota and attempt sending an email manually to rule out backend server restrictions.
In your Python script, temporarily comment out or remove the code blocks that attach files to the message.
Run the Python script again to send a plain text email. If it succeeds, the issue lies in how win32com is handling the attachment payload size.
Navigate to File > Office Account > About Outlook in your desktop client to identify your exact build version, ensuring you are not affected by a known bug in a specific release.
Clear Outlook Sync Issues and Local Cache
Clear local cache discrepancies that cause Outlook to report a full mailbox incorrectly to COM add-ins or scripts.
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If you frequently encounter caching, sizing, or COM automation issues with heavy desktop clients, consider trying WPS Office. It provides a lightweight, highly compatible, and free alternative to Microsoft Office, covering your document, spreadsheet, and presentation needs seamlessly.
Frequently Asked Questions
Why does my Python win32com script say my Outlook mailbox is full when it isn't?
This false error often occurs due to hidden sync issues, oversized attachments that exceed programmatic limits during base64 encoding, or a discrepancy between the local OST file size and the server mailbox quota.
Does this error happen on both classic and new Outlook desktop clients?
Automation via win32com relies on the classic Outlook COM architecture. The 'New Outlook' is based on web technologies and does not support traditional COM automation, meaning this specific error is limited to the classic desktop app.
How can I check my actual Outlook mailbox storage size?
In classic Outlook, go to File > Info and look at the Mailbox Settings section. Alternatively, log into Outlook on the web and check your storage under Settings > General > Storage.
