add a changes file for lmtp ratelimit

changes/next/lmtp-ratelimit

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+Description:
+
+Support user and lmtp rate limits in lmtp
+
+
+Config changes:
+
+This changes the behaviour of the maxlogins_per_user and maxlogins_per_host
+configuration options to limit each item per service, so 5 logins from a
+host for imap won't also limit http logins.
+
+It also limits lmtp connections for delivery to a locked mailbox,
+returning a 4xx code if additional connections are made that try to
+deliver to the same mailbox if there are already N connections waiting.
+
+
+Upgrade instructions:
+
+No config changes required, it's rare that you have more than one LMTP
+connection waiting on a mailbox so it's very unlikely that existing
+limits will affect lmtp - though you may want to double check that your
+limits still make sense given that each service now has a separate count.
+
+
+GitHub issue:
+
+If theres a github issue number for this, put it here.