Microsoft have just announced limits on SMTP Authenticated Submission client protocol.
Starting June 1, the following changes will begin rolling out for SMTP Authenticated Submission protocol:
- Sent email will now be stored in the Sent Items folder of the mailbox.
- Only three concurrent connections to our service per mailbox will be allowed. Additional connections will be rejected with the error: 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded.
So what is going to impact here. Well If you have configured single mailbox to send emails on multiple applications or devices then you have to think about here. Bulk emails cannot go through from Exchange online in upcoming days.
Microsoft has suggested to use multiple mailboxes in these scenario or sent emails in batches. Else you have to use a third party email server for sending bulk emails.