http://nguipxnkrp3qrzrlduhsatpcpwehnblzmlkc5ifiumxq4z5jlh4lwvid.onion/os_archive/os_OPSEC_357.html
Run postfix and dovecot on one server, and httpd on another shut off port 25 143 on the apache server, and depending on your use case shut off port 80, input on the mail server, etc.