You need to download the JavaMail API, and put the relevant jar files in your classpath.

javax.mail - Maven Repository

https://mvnrepository.com/artifact/javax.mail

JavaMail API (compat)2,679 usages. javax.mail » mailCDDLGPL. JavaMail API (compat). Last Release on Mar 5, 2013. 2. JavaMail API JAR488 usages.

javax.mail (Java(TM) EE 7 Specification APIs)

https://docs.oracle.com/javaee/7/api/javax/mail/package-summary.html

The javax.mail package defines classes that are common to all mail systems. The javax.mail.internet package defines classes that are specific to mail systems ...

java - package javax.mail and javax.mail.internet do not exist - Stack ...

https://stackoverflow.com/questions/6606529/package-javax-mail-and-javax-mail-internet-do-not-exist

When I compile a simple code that has the following 2 import statements: import javax.mail.*.

java - Sending mail error, javax.mail.MessagingException: Could not ...

https://stackoverflow.com/questions/32882595/sending-mail-error-javax-mail-messagingexception-could-not-connect-to-smtp-hos

Error is self explainatory: javax.mail.MessagingException: Could not connect to SMTP host: localhost, port: 25;. You have no SMTP server on localhost, ...