server side sender filter

email server senders filter

Not all email clients provide sophisticated ways to filter emails.
In these cases it is possible to act upstream.

The “Authorized senders” feature allows you to receive messages
only from the senders you have previously authorized
(you can also specify the entire domain, e.g. @example.com):

Authorized senders

All the regular messages will arrive as usual in your inbox.
All the spam messages will go to a different mailbox
or in the user’s “Junk” email folder of Microsoft 365 Exchange.

No emails will be lost.
You may read the discarded messages mailbox once or more a day.
You will save so much precious time.


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Subsections of server side sender filter

Microsoft 365 Exchange

Exchange Online


This configuration allows to correctly move mail
from UNauthorized senders to the user’s Junk email folder

The messages filtered by SpamStop app will arrive
with the following anti-spam headers and values:

X-Forefront-Antispam-Report: SFV:SKB

(message marked as spam by spam filtering
due to the sender’s email address or email domain
NOT present in the list of authorized senders)

The following Action must be activated:
Set the spam confidence level (SCL) of these messages to 6 (spam)

The default value of the SCLJunkThreshold parameter is 4, which means
an SCL of 5 or higher should deliver the message to the user’s Junk email folder.

  • In the In the Exchange admin center (EAC), go to Mail flow > Rules.

  • On the Rules page, select Add > Create a new rule in the dropdown list.

  • In the New rule page that opens, configure the following settings:


set rule conditions


Name: SpamStop

Apply this rule if: ‘X-Forefront-Antispam-Report-Untrusted’
message header matches: ‘SFV:SKB’

Do the following:
Modify the message properties
Set the spam confidence level “SCL” to: ‘6’


Save and Enable the rule.


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