国内线上走的是 smtp aliyun。海外也可以直接走smtp mailgun?
https://www.baeldung.com/spring-email
https://docs.spring.io/spring-boot/docs/2.0.x/reference/html/boot-features-email.html
https://docs.spring.io/spring-framework/docs/5.0.13.RELEASE/spring-framework-reference/integration.html#mail
https://documentation.mailgun.com/en/latest/user_manual.html#sending-via-smtp
https://app.mailgun.com/app/sending/domains/dm.tap.io/credentials