Microsoft Office 365 Email Limits

Account Limits

CSULB's Microsoft email service has a number of documented limits that, when exceeded, will result in account activity suspension for 24 hours. 

To discourage the delivery of unsolicited bulk messages, Microsoft has recipient limits that prevent users and applications from sending large volumes of email. These limits are applied per user to all outbound and internal messages. This feature is intended to limit the impact of email abuse.

Limit Type


Message size (Outlook, Outlook for Mac)

Note: Attaching a file to a message can increase the file size by as much as 30% when it is sent.

150 MB
Message size (OWA/Outlook Online) 112 MB
Message size (Outlook for iOS and Android) 33 MB
Number of maximum file attachments per message 250
File attachment size (Outlook, Outlook for Mac) 150 MB
File attachment size (OWA/Outlook Online) 112 MB
File attachment size (Outlook for iOS and Android) 33 MB
Mailbox Storage (user, shared, and resource mailboxes) 100 GB

Recipient Limit per message

Note: After the recipient rate limit is reached, messages can't be sent from the mailbox until the number of recipients that were sent messages in the past 24 hours drops below the limit. 

Recipient rate limit per day 10,000
Message rate limit per minute 30
Deleted Items folder retention period 30 days
Retention period for items removed from the Deleted Items folder 14 days
Junk Email folder retention period 30 days

Other Considerations

While the CSULB email system allows sending and receiving 150 MB email messages (including attachments), other external email systems continue to have their own limits on the receiving end when you are sending emails to them. Some common receiving limits are:

  • Gmail – 50 MB
  • – 50 MB
  • Yahoo – 25 MB
  • Peer universities – varies

Whenever possible, it is advised to use a file collaboration tool for large files, such as the campus-provided Microsoft OneDrive for Business which allows for file sizes larger than 150 MB.


For details and more information on other Microsoft limitations, please see Exchange Online Limits.

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Article ID: 127723
Wed 2/10/21 4:29 PM
Thu 5/13/21 3:51 PM