Simple Mail Transfer Protocol (SMTP) (edit)

SMTP Free

https://kinsta.com/knowledgebase/free-smtp-server/

SendGrid (SMTP Ports)

SendGrid accepts unencrypted and TLS connections on ports 25, 587, & 2525. You can also connect via SSL on port 465.

SMTP Tester

https://github.com/nekno/SMTP-Tester