There are several variables, which can be used for customization of notification:
Using variables to customize alerts and notifications
Symantec™ Mail Security for Microsoft® Exchange Server 2013/Server 2016 v7.5.5 Implementation Guide
Substitution of each variable does not work as expected.
That is, variables are replaced as follows:
%recipient% -> ecipient%n ("%r" is removed and "n" is appended)
%sender% -> sender%0 ("%" is removed and "0" is appended)
Outbreak notification is triggered by number of violation occurrence, that is, it's not dependent upon specific mail, therefore SMSMSE will not keep track of individual sender and recipient, which results in failure of variables ("%sender%"/"%recipient%") conversion.
This behavior is by design of product and one is not able use these variables for notification in outbreak.