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University Webmail and Email Support

University of Colorado Denver

Exchange Online Limits

​Email Limits in Office 365

Now that your mailbox lives in Office 365 you will receive a larger mailbox size and access to the entire Office 365 suite. Exceeding these limits will affect your ability to send and receive email messages. 

These limits cannot be increased or decreased.


Storage limits 
​Feature Office 365 Enterprise
​User mailboxes ​50 GB
​Archive mailboxes ​50 GB
​Shared mailboxes ​50 GB
​Resource mailboxes ​50 GB
​Site mailboxes ​5 GB
Capacity alerts

Note: There are three kinds of notifications when a mailbox is nearing, or at, capacity

  • Warning: You will receive an email warning that the mailbox is approaching the maximum size limit. This warning is intended to encourage the deletion of unwanted mail.
  • Prohibit Send: You will receive a prohibit-send notification email when the mailbox size limit is reached. You cannot send new messages until enough email is deleted to bring the mailbox below the size limit.
  • Prohibit Send/Receive: Exchange Online rejects any incoming mail when the mailbox size limit is reached, and sends a non-delivery report (NDR) to the sender. You will have the option to try resending the mail later. To receive messages again, the user must delete email until the mailbox is below the size limit.


​Feature Office 365 Enterprise
​Warning ​49 GB
​Prohibit Send ​49.5 GB
​Prohibit Send/Receive ​50 GB


Address book limits 
Feature ​Office 365 Enterprise
​Address list limit ​1000
​Offline address book (OAB) limit ​250
​Address book policies (ABP) limit ​250
​Global address lists limit ​250
Mailbox folder limits
Feature ​Office 365 Enterprise
​Maxium number of messages per mailbox folder ​1 million
​Warning for number of messages per mailbox folder ​900,000
​Maximum number of messages per folder in the Recoverable Items folder ​3 million
​Warning for number of messages per folder in the Recoverable Items folder ​2.75 million
​Maximum number of subfolders per mailbox folder ​1000
​Warning for number of subfolders per mailbox folder ​900
​Maximum folder hierarchy depth ​300
​Warning for folder hierarchy depth ​250
Message limits
​Feature ​Office 365 Enterprise
​Message size limit ​25 MB
​Message size limit - migration ​150 MB
​Subject length limit ​255 characters
​File attachments limit ​250 attachments
​File attachment size limit ​25 MB
​Multipart message limit ​250 parts
​Embedded message depth limit ​30 embedded messages
Recipient and sender limits
​Feature ​Office 365 Enterprise
​Recipient rate limit ​10,000 recipients per day
​Recipient limit ​500 recipients
​Message rate limit (SMTP client submission only) ​30 messages per minute
Retention limits
​Feature ​Office 365 Enterprise
​Deleted Items Folder ​No limit
​Retention period for items removed from the Deleted Items folder ​30 days
​Junk Email folder retention period ​30 days
Distribution group limits
​Feature ​Office 365 Enterprise
​Maximum number of distribution group members ​100,000 members
​Limit sending messages to large distribution group ​5,000 or more members
​Maximum message size for large distribution groups ​2 MB
Transport and Inbox rule limits
​Feature ​Offce 365 Enterprise
​Maximum number of transport rules ​300 rules
​Maximum size of an individual transport rule ​8 KB
​Character limit (in KB) for all regular expressions used in all transport rules ​20 KB
​Maximum number of recipients added to a message by all transport rules ​100 recipients
​Forwadee limit ​10 recipients
​Number of times a message is redirected ​1 redirection
Exchange ActiveSync limits
​Feature ​Office 365 Enterprise
​Exchange ActiveSync device limit ​100
​Exchange ActiveSync device deletion limit ​20
​Exchange ActiveSync file attachment limit ​25 MB

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