implement limits on outgoing messages for an account

by default 1000 messages per day, and to max 200 first-time receivers.
i don't think a person would reach those limits. a compromised account abused
by spammers could easily reach that limit. this prevents further damage.

the error message you will get is quite clear, pointing to the configuration
parameter that should be changed.

testdata/smtp/sendlimit/domains.conf

@@ -0,0 +1,9 @@
+Domains:
+	mox.example: nil
+Accounts:
+	mjl:
+		Domain: mox.example
+		Destinations:
+			mjl@mox.example: nil
+		MaxOutgoingMessagesPerDay: 4
+		MaxFirstTimeRecipientsPerDay: 2

testdata/smtp/sendlimit/mox.conf

@@ -0,0 +1,9 @@
+DataDir: data
+User: 1000
+LogLevel: trace
+Hostname: mox.example
+Postmaster:
+	Account: mjl
+	Mailbox: postmaster
+Listeners:
+	local: nil