No, the misunderstanding is yours. Postfix IS an SMTP server. SMTP servers connect to other SMTP servers to deliver mail. Your mail comes into an SMTP server just as it goes out an SMTP server.
ocelotpotpie meant that SMTP servers are designed to reliably queue mail and retransmit. If your mail server goes down, you don't lose mail. The senders will just try again later. Downtime on an SMTP server impacts delivery latency, not reliability.