Receiving Limits
The smtpd daemon can enforce a number of limits on incoming mail. The limits are configurable through several parameters in the main.cf file. You can limit the size of messages, the number of recipients for a single delivery, and the length of lines in a message. You can also limit the number of errors to allow from a single client before breaking off communications.
To limit the number of recipients for a single message, use the smtpd_recipient_limit parameter. The default
is 1,000 recipients, and it should be adequate for normal
operation.
The message_size_limit
parameter limits the size of any message your system will
accept. The default is 10 MB. If you have limited disk space or memory,
you might want to lower the value. On the other hand, if your users
commonly receive large attachments, you may have to increase it.
Increasingly frequent errors from the same client might indicate a problem or an attack. Postfix
keeps a counter of errors, and handles potential problem clients by
introducing delays with each error. The delays can help protect your
system from misconfigured or malignant clients. As the number of errors
increases so does the length of each delay. The length of the initial
delay is specified by smtpd_error_sleep_time with a default of one second. After the number of errors
exceeds the value set for smtpd_soft_error_limit , Postfix increases the delay by one second for every error, so that with each error, there is a slightly longer delay. ...