expand the task list in impleemntation details

not sure that's a good idea yet

policy/tpa-rfc-15-email-services.md

@@ -237,6 +237,28 @@ will start to degrade some time or before Q3 2022.
 
 Note: this task list must match the cost estimates below.
 
+### Implementation details
+
+ a. end-to-end deliverability monitoring involves:
+    
+    * actual delivery roundtrips
+    * block list checks
+    * DMARC/MTA-STS feedback loops (covered below)
+    
+    Implementation details are in [issue 40539][]
+
+ b. DMARC and MTA-STS reports analysis are also covered by [issue
+    40539][], but are implemented separately because they are
+    considered to be more complex (e.g. RBL and e2e delivery checks
+    are already present in Nagios)
+
+ c. DKIM will be generally implemented with OpenDKIM. it's currently
+    unclear how [ARC][] would be implemented, as the known
+    implementations ([OpenARC][] and [Fastmail's authentication
+    milter][]) are not packaged in Debian
+
+ d. TODO expand
+
 ## Timeline
 
 This timeline is a draft, and will be updated according to when this