It is possible that the Office 365 tenant of one of our clients is categorizing incoming emails as "spam." This occurs due to checks performed by the tenant, including the SPF check (which may fail in certain configurations).
We can bypass these checks by configuring our IPs within the Office 365 tenant. This enhanced filtering will prevent Mailinblack IPs from being flagged as illegitimate sending servers and avoid visible errors in email headers (such as SPF failures that occur when the system identifies a Mailinblack IP as the sending server for a domain like exemple.com, which doesn't include our IPs in its SPF record).
This configuration helps ensure proper categorization of emails in the inbox and prevents legitimate messages from being marked as spam.
Practical Link : https://learn.microsoft.com/fr-fr/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/enhanced-filtering-for-connectors
Log into the Admin interface
Go to the Admin Centers tab and click on Security
Go to Policies and Rules.
Then click on Threat policies.
Click next on Anti-spam.
In the new tab, open Connection filter policy and modify it.
Add our 5 IP ranges and exit while saving.
137.74.84.0/24
185.209.208.0/24
185.209.209.0/24
185.209.210.0/24
185.209.211.0/24
Return to the Admin Center and click on Exchange.
Go to the Mail Flow and Rules section and add a rule.
Name de rule and select “Apply this rule if The sender > The IP address is in any of these ranges or exactly matches”.
Add our IP addresses
137.74.84.0/24
185.209.208.0/24
185.209.209.0/24
185.209.210.0/24
185.209.211.0/24
Select “Do the following… Modify the message properties… > Set a message header”.
Modify the “Text inputs”:
Assign the first text to “X-MS-Exchange-Organization-BypassClutter”.
Assign the second text to “true”.
Add an action and select “Modify the message properties… > Set the spam confidence level (SCL)”.
Choose "set the spam confidence level" to -1
You can then click next and some specific settings for the rule will be suggested.
If you do not wish to add any specific rule, you can click next and save. Once saved, make sure the rule is enabled:
