There are various service limits for SUMail that are designed to protect the email service as a whole and detect anomalies such as compromised accounts. The following list breaks out limits that would most likely affect all email users, and for a complete list of service limits check the following link:
Exchange Online limits - Service Descriptions | Microsoft Docs
All active Students, Staff, and Faculty are on an E3 license plan. Alumni are E1, and inactive users aren't licensed. Shared/Resource mailboxes also do not receive a license since they are not directly logged into.
|Message rate limiting||30 email submissions per minute, per mailbox|
There are no overrides for this.
No sent messages are lost or denied, they will sit in users outbox until the next minute and send as many as possible.
|Recipient rate limiting||10000 recipients every 24 hours, per mailbox|
Specifies the total number of email addresses which can be mailed to in a 24 hour period, whether unique or not.
|Receive rate limit||3,600 messages per hour, per mailbox|
|User mailbox storage||100gb (active staff/faculty/student mailboxes)|
|Shared/resource mailbox storage||50gb|
|Alumni mailbox storage||50gb|
|Archive mailbox storage||Unlimited||Available only for E3 licensed mailboxes|
|Messages per mailbox folder||1 million|
|Folders per mailbox||10,000|
|Message size limit||50mb||This includes attachments, but does not account for mime encoding or client limits.|
|Junk mail folder retention||30 days|
|Deleted Items retention||90 days|
|Inbox Forwarding Rule limit||10 Forwardee limit||The maximum number of recipients that can be configured for an inbox or transport rule with a redirecting action.|