The purpose of the limit applied by Microsoft is to combat spam and mass-mailing worms or virus. These limits help to protect the health of Microsoft systems and keep users safe.
A. Sending limits
Email Sending limits apply to:
| Limit | Microsoft 365 |
| 1. Number of recipients per 24 hours, or Recipient rate limit (starting from 1 April 2026) | (a)10,000 recipients per day (in a 24-hour window), including both internal recipients & external receipts. If you are sending an Email to another Email domain within HKU, e.g. staff account (@hku.hk domain) sending Email to student account (@connect.hku.hk) will be considered as sending to external recipients |
| (b) 2,000 external recipients per day, and it is a sub-limit of 10,000 recipients limit per day (in a 24-hour window) mentioned in (a) Example: – You use a cloud-hosted mailbox to send to 1,000 external recipients and 2,000 internal recipients at 6:00AM on Day 1, for a total of 3,000 recipients. You then send to another 1,000 external recipients at 8:00AM on Day 1. Because you sent to 2,000 external recipients, you will be blocked from sending to external recipients until 6:00AM on Day 2. During this period, you are able to send to up to 6,000 internal recipients, but for this example, let’s assume that you don’t. – At 6:00AM on Day 2, the 1,000 external recipients and 2,000 internal recipients sent at 6:00AM on Day 1 will no longer count toward the 24-hour limit calculation. Thus, from 6:00-8:00AM on Day 2, you can send to up to 9,000 total recipients (e.g., 9,000 internal or 8,000 internal plus 1,000 external). – If you don’t send to any internal or external recipients during this period, then at 8:00AM on Day 2 you can again send to 10,000 recipients total with up to 2,000 of them being external recipients. | |
| 2. Number of recipients per email message | Maximum 200 recipients |
| 3. Number of Email messages per minute (SMTP client submission only) | Maximum 30 messages per minute |
Remarks:
(1) If sending Email to distribution groups:
- 1 group is counted as 1 recipient for groups stored in Organization’s address book:
- Members of the group are counted as individually for groups stored in Contacts folder of individual mailbox
(2) Sending from another mailbox via permission is counted for the delegate with rights and not the mailbox that is being sent from.
B. Receiving limits
Email Receiving limits apply to:
- Number of messages that a user account or email group can receive per hour.
- Messages received from internal senders, from internet and from on-premise mail servers.
When the receiving limit has been exceeded:
- Any emails sent to that mailbox will receive a non-delivery report stating that the mailbox has exceeded the maximum delivery threshold.
- – After 1 hour, the limit will refresh and the mailbox will be able to receive messages.
| Limit | Microsoft 365 |
| Receiving Email Limit | 3600 messages per hour |
| Messages received from a single sender (Sender-recipient pair limit) | 33% of receiving limit (i.e. 1200 messages per hour) |
C. The maximum size of email you can send and receive at HKU
HKUCC-COM (@hku.hk for staff) | 30MB |
CONNECT (@connect.hku.hk for students and alumni connect users) | 25MB |
Graduate.hku.hk (@graduate.hku.hk for graduates) | 10MB |