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Microsoft 365 - What are the limitations for message size, recipient number, and mailbox storage?

The specifications below outline the mailbox storage capacity limits as well as message and recipient size quotas associated with Office 365. Email usage data collected by the Administrative Excellence Phase II team suggest that only a very small percentage of campus users have email capacity needs beyond these limitations.

Important: The limits discussed below are set by Microsoft and cannot be controlled by UW-Madison Microsoft 365 administrators. For example, if you exceed the sent message daily limit, your account will be blocked from sending any further messages for a period of 24 hours.

Mail size quotas / caps:

Outgoing Messages

Outlook on the web
25 MB
Outlook for Windows **
20 MB
Other clients
(Thunderbird, Apple Mail, SMTP relay, etc.) ***
25 MB
Incoming messages

All Clients
25 MB

** Client limits are set by Microsoft and cannot be altered by the UW-Madison Microsoft 365 team.

*** The Office 365 Technical Team is still in the process of determining whether there will be additional message size limitations for mobile devices. Some mobile clients may impose additional message or attachment size limits. These limitations can vary by device, client or settings selected.

These limits include all attachments, as well as the message itself. If an incoming message larger than 25 MB is sent to Office 365 recipient, the sender will receive a notification that the delivery has failed.

Other limits / caps:

Max email mailbox storage
See quota article
Max recipients per message ****
Max recipients per day
Maximum number of distribution group members
Limit sending messages to large distribution groups
5,000 or more members
Maximum message size for large distribution groups
2 MB
Number of Attachments *****
can not exceed 256
Uncompressed Attachment size *****
can not exceed 35MB
File Archives/Zips *****
can not contain more than 256 files
File Archives/Zips *****
can not have a file depth greater than 20

**** Individuals who are eligible for Duo Multi-factor authentication but who have not yet enrolled are limited to 5 recipients per message. See MFA-Duo - How to Enroll for MFA Duo for your NetID Login Account for more information on how to enroll for Duo MFA.

***** Any message that exceeds these limits, inbound or outbound, will be returned to sender with the following error message: "552 5.3.4 a message attachment exceeds local limits".

Review Microsoft Exchange Online Limits and Microsoft Email non-delivery reports and SMTP errors in Exchange Online documentation for a more thorough and up-to-date details.


When sharing data (attachments), due to security and ease of use - email is not the best tool to use. The UW-Madison Microsoft 365 service team recommends using a cloud based file sharing solution.

Additional Resources

Keywordsmicrosoft office365 o365 m365 microsoft 365 quotas email limitations attachments limits recipients max maximum outlook clients relay smtp proofpoint   Doc ID27922
OwnerO365 S.GroupMicrosoft 365
Created2013-01-07 14:46:11Updated2023-04-04 14:31:40
SitesDoIT Help Desk, Microsoft 365, Wisc Account Admin
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