Configuring the email-ext plugin using the CasC plugin, the smtpHost is ignored when also setting the smtpUsername / smtpPassword authentication fields.

The Email Extension plugin can be set up 'hands on' in the UI, so using the necessary fields described in the CasC documentation, I set up something like this:

  extendedEmailPublisher:     smtpServer: smtp.server.com
    smtpUsername: alerts@company.com
    smtpPassword: ${SMTPpass}
    useSsl: true
    charset: UTF-8
    defaultContentType: text/plain
    defaultSubject: "Build $BUILD_NUMBER - $BUILD_STATUS"
    defaultBody: "Check console output at $BUILD_URL"

But adding the smtpUsername / smtpPassword fields seems to cause the 'smtpServer' value to be ignored completely (it's not ignored when the smtpUsername / smtpPassword fields are not present). Instead, the 'smtpHost' value from the Mailer plugin configuration is used in its place

In order to get SMTP authentication working for the Email Extension plugin, I used this setup:

  mailer:     smtpHost: smtp.server.com
    useSsl: true
    charset: UTF-8
  extendedEmailPublisher:     smtpUsername: alerts@company.com
    smtpPassword: ${SMTPpass}
    defaultContentType: text/plain
    defaultSubject: "Build $BUILD_NUMBER - $BUILD_STATUS"
    defaultBody: "Check console output at $BUILD_URL"

It's not ideal that part of the configuration lives in each plugin though (the mailer plugin does not support SMTP auth fields - JENKINS-55109)

See also: https://github.com/jenkinsci/configuration-as-code-plugin/issues/772