Is Office365 large Group Email Blocking Related to CSF?

[webdsn-sanbis]

Recently reported by users
When using office365 to send large group mail to the host, there is a bounced message

There are two types of error messages currently appearing:

7/13/2022 5:30:19 AM - Server at PR3PR09MB4265.eurprd09.prod.outlook.com returned '550 5.4.318 Message expired, connection reset (SuspiciousRemoteServerError)(450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError))'
7/13/2022 5:18:06 AM - Server at xxxxxxxx (xx.xx.xx.xx) returned '450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError)'

Your message wasn't delivered. Despite repeated attempts to deliver your message, a connection to the remote server was closed abruptly.

Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that your email system is unable to connecto their email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.

For Email Admins
This often indicates the recipient's firewall SMTP fixup setting or other firewall settings are preventing the SMTP protocol negotiation from succeeding.

For more information and tips to fix this issue see this article: http://go.microsoft.com/fwlink/?LinkId=389361.

Since there is no relevant information in the mail log
Don't know if it's possible to get blocked by CSF before entering the host

[kiasujones]

To determine if this bounced message is directly related or unrelated to the CSF software, you need to systematically attempt to replicate this bounced message with 2 distinct scenarios:
i. with CSF enabled
ii. with CSF disabled

Once you do that, the situation should become clearer.