Is Postfix the same thing as Sendmail?

Sendmail is a different (and much older) program from Postfix. However for every mail server to succeed in the Unix environment, a sendmail binary (with some of the expected command line options) must be provided.

EDIT: See for example the manual page for the sendmail program provided by Postfix

Postfix and sendmail are different implementations of Mail Transfer Agents which are using Simple Mail Transfer Protocol (SMTP) for email transport over the Internet.

Postfix first released in 1998 intended to be an alternative to the widely used Sendmail MTA which was used widely since 1982.

Here are the summaries of all 4 popular mail agents:

And their comparison table:

^{Source: shearer}

For full comparison, check: MTA Comparison at shearer.org

Sendmail and Postfix are indeed both Mail Transfer Agents (MTAs).

Postfix is quite a bit easier for a new admin to set up, and has some nice features that integrate well with the mail store end of the process (Cyrus POP/IMAP, Dovecot, etc.).

If you're not actually going to be accepting email incoming into that box and it's getting forwarded to another system, something lighter such as Exim may be a better choice.

Architecture: The most common difference between these two is architecture. Postfix has a modular architecture composed of many independent small executables. It provides multiple options, parameters, and features. In contrast, Sendmail has a monolithic design that uses a single process always running at the backend.

Performance: Sendmail is not great in terms of its performance, making it a poor competitor. Postfix has a queue manager that handles the queuing much more quickly.

Source: https://linuxhint.com/postfix_vs_sendmail/