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:
Remarks:
(1) If sending Email to distribution groups:
| Limit | Microsoft 365 |
| 1. Number of recipients per 24 hours, or Recipient rate limit (starting from 1 April 2026) | 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 |
| 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 |
- 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
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 |