`local_destination_concurrency_limit`

This parameter limits the number of parallel deliveries to the same local recipient.

The default limit is taken from the `default_destination_concurrency_limit` parameter. Postfix documentation recommends a low limit of 2, just in case someone has an expensive shell command in a `.forward` file or in an alias (e.g., a mailing list manager). You don't want to run lots of those.