Maybe it should be enabled somewhere ? Is that an addon, a marketplace item ? I'm a bit lost on how it's integrated. Can't find much infos in the doc.

I thought it would be listed here but nope:
https://help.smartertools.com/SmarterMail/Current/Topics/Installation/Addons
Neither here:
https://help.smartertools.com/SmarterMail/Current/Topics/Installation/SpamVirusIntegration

Try looking for entries of "[SpamFoo Client]" in the delivery log, see if you notice any errors or indicators of issues. You can also try looking for the running process "spamfoo-client" running on the machine. If neither of those are the case there is a debug log id (also set in the troubleshooting area) you can add '20251120' to the debug log id's list and another log will show up with a lot more information, I'd restart after flipping these logs on incase there are things logged only during spinup that may indicate an issue.

Also just so I can start a list of things, if those seeing issues can also include some information about your environment like what OS, is is a fairly 'default' install or any other interesting details, could start to find correllations. 

windows 2022 with standard installation. All other Spam Filters are wotking fine.

I see a lot of theese errors in DELIVERY LOG:

[2026.06.28] 10:11:05.651 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:11:14.268 [SpamFoo Client] Re-attempting start of SpamFoo in 19,4009434 seconds.

[2026.06.28] 10:11:14.269 [SpamFoo Client] [18565882] Error: Could not start SpamFoo.

[2026.06.28] 10:11:29.296 [SpamFoo Client] Re-attempting start of SpamFoo in 4,3731705 seconds.

[2026.06.28] 10:11:29.301 [SpamFoo Client] [18565885] Error: Could not start SpamFoo.

[2026.06.28] 10:11:38.684 [SpamFoo Client] Test connection to SpamFoo Daemon failed after 5 attempts. 

[2026.06.28] 10:11:38.684 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:11:38.684 [SpamFoo Client] [presidente@prolocolentiai.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:11:38.684 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:11:38.684 [SpamFoo Client] [info@legatoriaprimia.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:11:38.795 [SpamFoo Client] Attempting to start SpamFoo Daemon @ C:/Program Files (x86)/SmarterTools/SmarterMail/Service/SpamFoo/bin/spamfoo-client.exe [WD: C:/Program Files (x86)/SmarterTools/SmarterMail/Service/SpamFoo] [DataDir: C:/Program Files (x86)/SmarterTools/SmarterMail/Service/SpamFoo/data]

[2026.06.28] 10:11:38.797 [SpamFoo Client] SpamFoo Daemon Started.

[2026.06.28] 10:11:38.797 [SpamFoo Client] TestConnectionToSpamFoo

[2026.06.28] 10:11:38.798 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:11:38.798 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:11:40.687 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:11:40.688 [SpamFoo Client] [segreteria@prolocolentiai.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:11:40.794 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:12:13.875 [SpamFoo Client] Test connection to SpamFoo Daemon failed after 5 attempts. 

[2026.06.28] 10:12:13.875 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:12:13.875 [SpamFoo Client] [info@legatoriaprimia.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:12:13.875 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:12:13.875 [SpamFoo Client] [segreteria@prolocolentiai.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:12:13.987 [SpamFoo Client] Attempting to start SpamFoo Daemon @ C:/Program Files (x86)/SmarterTools/SmarterMail/Service/SpamFoo/bin/spamfoo-client.exe [WD: C:/Program Files (x86)/SmarterTools/SmarterMail/Service/SpamFoo] [DataDir: C:/Program Files (x86)/SmarterTools/SmarterMail/Service/SpamFoo/data]

[2026.06.28] 10:12:13.990 [SpamFoo Client] SpamFoo Daemon Started.

[2026.06.28] 10:12:13.990 [SpamFoo Client] TestConnectionToSpamFoo

[2026.06.28] 10:12:13.990 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:12:13.990 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:12:15.874 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:12:15.874 [SpamFoo Client] [presidente@prolocolentiai.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:12:15.994 [SpamFoo Client] ClassifyTabs: _testConnectionTask=running, ActiveServiceCapabilities=False

[2026.06.28] 10:12:49.027 [SpamFoo Client] Test connection to SpamFoo Daemon failed after 5 attempts. 

[2026.06.28] 10:12:49.027 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:12:49.027 [SpamFoo Client] ClassifyTabs: after await, ActiveServiceCapabilities=False

[2026.06.28] 10:12:49.027 [SpamFoo Client] [presidente@prolocolentiai.it] Error: Classification error: SpamFoo service is not available.

[2026.06.28] 10:12:49.027 [SpamFoo Client] [info@legatoriaprimia.it] Error: Classification error: SpamFoo service is not available.

I upgraded a second server that is on Windows 2019. Same issue...

The SpamFoo service isn't working

At least you got the services responding after the upgrade.... You lucky fellow!