Overview
Microsoft Exchange Online is the primary service for sending and receiving messages. There are quite a few restrictions on Exchange mailboxes but the primary limits are related to mailbox size and sending limits. Microsoft has published a number of default service limits here: https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits including a section on "Receiving and sending limits".
Note: the receiving and sending limits can be limited to a specific scope, for example, non-MFA enforced users may have a very restrictive sending limit to avoid compromised accounts sending mass email.
Limits
What is the default receiving limit for a single user?
3,600 message per hour. Typically this limit does not affect normal user accounts.
What is the default sending limit for a single user?
- Employees: 500 recipients per hour, with 1,000 recipients total per 24 hours
- Students: 250 recipients per hour, with 500 recipients total per 24 hours
- For both students and employees, there is a 500 recipients per message limit
Additional information about recipient limits is available here: https://support.uidaho.edu/TDClient/40/Portal/KB/ArticleDet?ID=2295
What is the default mailbox size limit?
Active employees and students they are assigned an "A3" license which allows them to have up to 100 GB in a single mailbox. It is also possible to add an Archive mailbox that has 1.5 TB of storage. For all other accounts the mailbox is limited to 50 GB with a 50 GB Archive mailbox.
What is the default maximum message size limit?
A single email message can be up to 150 MB in size although there are limits which restrict this to 50 MB for outbound email. It is generally recommended to not attach more than 33 MB of content in a single message. If larger files are needed a link to OneDrive should be sent instead of the file.
Additional Resources
Microsoft has additional service limit details available here: https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits