Following warn messages are always logged by using camel-mail component.
fuse.log

2019-04-25 12:42:09,806 | WARN | 23 - scheduler://foo | j.mail | 21 - com.sun.mail.javax.mail - 1.6.1.redhat-1 | expected resource not found: /META-INF/javamail.default.providers
2019-04-25 12:42:09,807 | WARN | 23 - scheduler://foo | j.mail | 21 - com.sun.mail.javax.mail - 1.6.1.redhat-1 | expected resource not found: /META-INF/javamail.default.address.map

part of camel route

<to id="_to1" uri="smtp:smtp.host:smtp.port?password=smtp.password&username=smtp.username"/>

I confirmed that javamail.default.providers and javamail.default.address.map are packaged in javax.mail-1.6.1.redhat-1.jar and also in my reproducer app.

I guess the reason is a class loader for camel-mail(org.apache.camel.component.mail.DefaultAuthenticator) is used for Class.getResourceAsStream(), and could not find one in javax.mail-1.6.1.redhat-1.jar nor reproducer app.
Please check the constructor for javax.mail.Session and Session.getResourceAsStream() method.