Outlook: Outgoing Email issue in application servers due to concurrent connections limit issue

(1) Current outgoing Email setup in our customer's application servers:

We have configured the Outlook Email credentials in our customer's application servers for outgoing Email functionality:

That is, something like this:

 SMTP Host: smtp.office365.com

 SMTP username: <EMAIL-ID>

 SMTP password: ****

There are 3 servers running, but currently only 2 servers are used for sending out mails using the outlook SMTP credentials.

(2) Issue:

Recently, intermittently we are facing the issue, in which emails are blocked due to the limit imposed by Outlook.

When checked in application logs, we can see the following error:

com.sun.mail.smtp.SMTPSendFailedException: 432 4.3.2 Concurrent connections limit exceeded. Visit https://aka.ms/concurrent_sending for more information. [Hostname=****.PROD.OUTLOOK.COM]

When checked in https://aka.ms/concurrent_sending for the above-mentioned code, it says as follows:

”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

(3) My Query/request regarding this:

Kindly let us know how we can resolve this intermittent errors happening.

As you can see, at any point of time there are only two concurrent connections ( only 2 servers are used for sending the emails).

We confirm that our application is making a single connection only at a

time.

Kindly let me know if any further details required from my end.