Exim can be configured to use SpamAssassin.

Perhaps the most useful way to integrate SpamAssassin is in ACLs. This way, spam can be rejected at SMTP time, so you have no worries about generating bounce messages (collateral spam), discarding email, or users failing to find false positives. False positives should be bounced back to the sender by the SMTP client.

https://maretmanu.bobu.eu/homepage/inform/exim-spam.php#spam

shows one way to include SpamAssassin.