SharePoint Online uses throttling to maintain optimal performance and reliability of the SharePoint Online service. Throttling limits the number of API calls or operations within a time window to prevent overuse of resources.
New throttling limit for concurrent connections that submit messages
The service has various limits to prevent abuse and to ensure fair use. An additional limit is being added. Under the new limit, up to three concurrent connections are allowed to send email messages at the same time. If an application tries to send more than three messages at the same time by using multiple connections, each connection will receive the following error message:
Message rate limit: Message rate limits determine how many messages a user can send from their Exchange Online account within a specified period of time. This limit helps prevent over consumption of system resources by a single sender. If a user submits messages at a rate that exceeds the limit via SMTP client submission, the messages will be rejected and the client will need to retry.
Bryan Baca
SharePoint
https://learn.microsoft.com/en-us/sharepoint/dev/general-development/how-to-avoid-getting-throttled-or-blocked-in-sharepoint-online
From Microsoft:
SharePoint Online uses throttling to maintain optimal performance and reliability of the SharePoint Online service. Throttling limits the number of API calls or operations within a time window to prevent overuse of resources.
Mail (SMTP)
https://learn.microsoft.com/en-us/exchange/troubleshoot/send-emails/smtp-submission-improvements
From Microsoft:
New throttling limit for concurrent connections that submit messages
The service has various limits to prevent abuse and to ensure fair use. An additional limit is being added. Under the new limit, up to three concurrent connections are allowed to send email messages at the same time. If an application tries to send more than three messages at the same time by using multiple connections, each connection will receive the following error message:
432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded
Additional throttling limits for the SMTP Authentication protocol are:
https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits
Sending limits
Message rate limit: Message rate limits determine how many messages a user can send from their Exchange Online account within a specified period of time. This limit helps prevent over consumption of system resources by a single sender. If a user submits messages at a rate that exceeds the limit via SMTP client submission, the messages will be rejected and the client will need to retry.