JMC 7.0.0 is intended to work with JDK 11. For
javax.activiation is an EE module and its not shipped any more with JDK 9 onwards. In order to use mail feature in JMC 7.0.0 we need to use mail.api which requires javax.activation.
javax.activation:1.2.0 is available in maven central and it works with mail.api:1.6.0.