SELinux is based on the least level of access required for a service to run. Services can be run in a variety of ways; therefore, you must tell SELinux how you are running services. The following Boolean allows you to tell SELinux how you are running Postfix:
allow_postfix_local_write_mail_spool
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Having this Boolean enables Postfix to write to the local mail spool on the system. Postfix requires this Boolean to be enabled for normal operation when local spools are used.