Enforcing some sort of limit on message size is always a good idea. Today's bandwidth and connection standards (especially regarding clients) don't make it as essential as it was in the past, but having no limits at all is always a bad practice that could lead to an easy DoS condition.
Every MTAallows some sort of message limiting. Sendmail uses the MaxMessageSize variable, whereas Postfix provides the message_size_limit setting.
define(~confMAX_MESSAGE_SIZE,,~5242880') # size in bytes, 5 MB
The same philosophy of not allowing unlimited resources for your emails can be applied to connection parameters, like the rate and number of connections per client as well as the maximum number of allowed children. Both Sendmail and Postfix provide many options for the task; enable them using conservative values and then tune them when necessary:
define(~confMAX_DAEMON_CHILDREN',~500')dnl define(~confQUEUE_LA',~20')dnl define(~confREFUSE_LA',~30')dnl define(~confC0NNECTI0N_RATE_THR0TTLE',~30')dnl FEATURE('conncontrol')dnl FEATURE('ratecontrol', ,~terminate')dnl ClientRate: 10
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