-This option names a file or DBM database in which a record of each
-'To:' recipient is kept when the message is specified by the transport.
-*Note*: This does not apply to 'Cc:' or 'Bcc:' recipients.
-If %once_file_size% is not set, a DBM database is used, and it is allowed to
-grow as large as necessary. If a potential recipient is already in the
-database, no message is sent by default. However, if %once_repeat% specifies a
-time greater than zero, the message is sent if that much time has elapsed since
-a message was last sent to this recipient. If %once% is unset, the message is
-always sent.
-
-If %once_file_size% is set greater than zero, it changes the way Exim
-implements the %once% option. Instead of using a DBM file to record every
-recipient it sends to, it uses a regular file, whose size will never get larger
-than the given value. In the file, it keeps a linear list of recipient
-addresses and times at which they were sent messages. If the file is full when
-a new address needs to be added, the oldest address is dropped. If
-%once_repeat% is not set, this means that a given recipient may receive
-multiple messages, but at unpredictable intervals that depend on the rate of
-turnover of addresses in the file. If %once_repeat% is set, it specifies a
-maximum time between repeats.
+This option names a file or DBM database in which a record of each 'To:'
+recipient is kept when the message is specified by the transport. *Note*: This
+does not apply to 'Cc:' or 'Bcc:' recipients.
+
+If %once% is unset, or is set to an empty string, the message is always sent.
+By default, if %once% is set to a non-empty file name, the message
+is not sent if a potential recipient is already listed in the database.
+However, if the %once_repeat% option specifies a time greater than zero, the
+message is sent if that much time has elapsed since a message was last sent to
+this recipient. A setting of zero time for %once_repeat% (the default) prevents
+a message from being sent a second time -- in this case, zero means infinity.
+
+If %once_file_size% is zero, a DBM database is used to remember recipients, and
+it is allowed to grow as large as necessary. If %once_file_size% is set greater
+than zero, it changes the way Exim implements the %once% option. Instead of
+using a DBM file to record every recipient it sends to, it uses a regular file,
+whose size will never get larger than the given value.
+
+In the file, Exim keeps a linear list of recipient addresses and the times at
+which they were sent messages. If the file is full when a new address needs to
+be added, the oldest address is dropped. If %once_repeat% is not set, this
+means that a given recipient may receive multiple messages, but at
+unpredictable intervals that depend on the rate of turnover of addresses in the
+file. If %once_repeat% is set, it specifies a maximum time between repeats.