The "Smtp destination concurrency limit" option limits the number of parallel deliveries to the same destination via the smtp delivery agent.
The default limit is the "Default destination concurrency limit" option. It is probably safer to limit the concurrency to 10. This option sets the "smtp_destination_concurrency_limit" postfix variable.
The "Smtp destination recipient limit" option limits the number of recipients per delivery via the smtp delivery agent.
The default is taken from the "Default destination recipient limit" option. This option sets the "Smtp destination recipient limit" postfix variable. This option sets the "smtp_destination_recipient_limit" postfix variable.
The "Default destination recipient limit" option specifies a default limit on the number of recipients per message delivery. This is the default limit for delivery via SMTP, via the local delivery agent and via the pipe mailer. This option sets the "default_destination_recipient_limit" postfix variable.