implement only monitoring dns blocklists, without using them for incoming deliveries

so you can still know when someone has put you on their blocklist (which may
affect delivery), without using them.

also query dnsbls for our ips more often when we do more outgoing connections
for delivery: once every 100 messages, but at least 5 mins and at most 3 hours
since the previous check.

config/doc.go

@@ -239,9 +239,12 @@ See https://pkg.go.dev/github.com/mjl-/sconf for details.
 				# accept/reject decision. This prevents sending IPs of all communications to the
 				# block list provider. If any of the listed DNSBLs contains a requested IP
 				# address, the message is rejected as spam. The DNSBLs are checked for healthiness
-				# before use, at most once per 4 hours. Example DNSBLs: sbl.spamhaus.org,
-				# bl.spamcop.net. See https://www.spamhaus.org/sbl/ and https://www.spamcop.net/
-				# for more information and terms of use. (optional)
+				# before use, at most once per 4 hours. IPs we can send from are periodically
+				# checked for being in the configured DNSBLs. See MonitorDNSBLs in domains.conf to
+				# only monitor IPs we send from, without using those DNSBLs for incoming messages.
+				# Example DNSBLs: sbl.spamhaus.org, bl.spamcop.net. See
+				# https://www.spamhaus.org/sbl/ and https://www.spamcop.net/ for more information
+				# and terms of use. (optional)
 				DNSBLs:
 					-
 
@@ -1198,6 +1201,14 @@ See https://pkg.go.dev/github.com/mjl-/sconf for details.
 			MinimumAttempts: 0
 			Transport:
 
+	# DNS blocklists to periodically check with if IPs we send from are present,
+	# without using them for checking incoming deliveries.. Also see DNSBLs in SMTP
+	# listeners in mox.conf, which specifies DNSBLs to use both for incoming
+	# deliveries and for checking our IPs against. Example DNSBLs: sbl.spamhaus.org,
+	# bl.spamcop.net. (optional)
+	MonitorDNSBLs:
+		-
+
 # Examples
 
 Mox includes configuration files to illustrate common setups. You can see these