<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-mail</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

pom.xml里的
<!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-mail -->
<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-mail</artifactId>
   <version>2.1.3.RELEASE</version>
</dependency>

import导入的
import org.springframework.mail.SimpleMailMessage;

可以看一下有没有导入错的