Microsoft Exchange Online is a cloud-based messaging solution that delivers e-mail, calendar, contacts, and tasks from PCs, the web, and mobile devices. This service is fully integrated with Azure Active Directory (AAD). The following limits of the service apply to users.
User Mailbox
Active Employees and Students
For active employees (faculty and staff) and students the maximum size of a user mailbox is 100 GB. This mailbox quota is mandated by Microsoft and cannot be increased. If more space is needed, archiving should be enabled. In addition to the main mailbox, each user can have on-line archive mailbox that cannot exceed 1.5 TB for active employees (faculty and staff) and students. Each user initially receives 100 GB of storage in the archive mailbox. When auto-expanding archiving is turned on, additional storage is automatically added when the 100 GB storage capacity is reached. Additional storage space is added incrementally until the archive storage capacity reaches 1.5 TB.
Retirees and Alumni
For retired employees and alumni the maximum size of a user mailbox is 50 GB. This mailbox quota is mandated by Microsoft and cannot be increased. If more space is needed, archiving should be enabled. In addition to the main mailbox, each user can have on-line archive mailbox that cannot exceed 50 GB for retired employees and alumni.
Shared Mailbox
The maximum size of a shared mailbox is 100 GB.
Message Size
The values for maximum sent size is set to 35 MB and for received to 36 MB for all users. This can be adjusted by the administrator as need on a per user basis.
Distribution Lists
Maximum number of distribution group members is 100,000. The total recipient count is determined after distribution group expansion.
If a message is sent to a distribution list that contains 5,000 or more recipients, the message size can't exceed 25 MB size limit. If the message size exceeds the limit, the message isn't delivered, and the sender receives a non-delivery report (NDR).
Limit sending messages to large distribution group is 5,000 or more members. Distribution groups that contain the number of members specified by this limit must have delivery management or message approval options configured. Delivery management specifies a list of senders who are allowed to send messages to the distribution group. Message approval specifies one or more moderators who must approve all messages sent to the distribution group.
Receiving Limits
Receiving limits are applied to combat spam and mass-mailing worms or viruses and is set at 3,600 messages per hour from any and all sources. When the receiving limit has been exceeded on a mailbox, messages sent to the mailbox from the internet or on-premises senders will be returned to the sender in a non-delivery report (also known as an NDR or bounce message) stating that the mailbox has exceeded the maximum delivery threshold. Messages from internal senders will count against the limit, but won't be blocked if the receiving limit has been exceeded. After an hour, the limit will refresh and the mailbox will be able to receive messages.
The number of messages per hour from a single sender is set at 33% of receiving limit which comes down to 1,188 messages per hour. This is in place to protect against single-sender mail storms.
Sending Limits
Sending limits are applied to combat spam and mass-mailing worms or viruses and apply to the:
- number of recipients
- number of messages
- number of recipients per message
... that a user can send from their Exchange Online account.
The maximum number of recipients allowed in the To:, Cc:, and Bcc: fields for a single email message is set per user at 500 recipients. This can be increased by the admin to up to 1,000.
Recipient rate limit is set at 1000 recipients per day, see this article for details. 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. For example, a user sends an email message to 500 recipients at 09:00 AM, then sends another message to 250 recipients at 10:00 AM, and then sends another message to 250 recipients at 11:00 AM, hitting the limit of 1000 messages. The user won't be able to send messages again until 09:00 AM the next day. Sending from another mailbox, like a shared mailbox, is counted for the shared mailbox that the message is being sent from and not the user sending the message as long as the shared mailbox was added to Outlook as a new account, not as a shared mailbox.
Message rate limit determines how many messages a user can send from their Exchange Online account within a specified period of time and is set at 30 message per minute.
NOTE: For distribution groups stored in an organization's address book, the group is counted as one recipient. For distribution groups stored in the Contacts folder of a mailbox, the members of the group are counted individually.
Other Limits
For more information, please see Microsoft Support article Exchange Online limits