Strapi is an application that runs on a web server, which is using Node.
For emails to work, it requires an email server to manage the handshakes between itself and another mail server. Sendmail is an integration between the web server and the email server.
The easiest solution in my opinion, if your web server doesn’t have an email server, is to use an existing email server via SMTP, AWS SNS or the equivalents on other cloud platforms.
To help you further, where will you host your Strapi server?