Internationalised mail name handling.
RFCs 6530, 6533, 5890
-Compile with libidn.
+Compile with EXPERIMENTAL_INTERNATIONAL and libidn.
+
+Main config option smtputf8_advertise_hosts, default '*',
+a host list. If this matches the sending host and
+accept_8bitmime is true (the default) then the ESMTP option
+SMTPUTF8 will be advertised.
+
+If the sender specifies the SMTPUTF8 option on a MAIL command
+international handling for the message is enabled and
+the expansion variable $message_smtputf8 will have value TRUE.
+
+The option allow_utf8_domains is set to true for this
+message, but all DNS lookups are converted to a-label form.
+
+Log lines and Received-by: header lines will aquire a "utf8"
+prefix on the 'with' element, eg. utf8esmtp.
+
+Expansion operators:
+ ${utf8_domain_to_alabel:str}
+ ${utf8_domain_from_alabel:str}
+ ${utf8_localpart_to_alabel:str}
+ ${utf8_localpart_from_alabel:str}
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