Custom Workflow step for sending mail

You can already use mailer-plugin from a flow:

step $class: 'hudson.tasks.Mailer', recipients: '...'

However this just runs the canned functionality of Jenkins build mail notification: mail is sent only for unstable/failed builds, with a predetermined subject, etc. (Use catchError to send even after an earlier step throws an error.)

email-ext-plugin integration would add a bunch more options, but still this would be focused on picking up existing behaviors related to build status: different stability triggers, changelogs, etc.

For some cases you would prefer to unconditionally send a message with a given subject and a given body to a given list of recipients and that is that. This is best done with a special step:

mail to: '...', cc: '...', subject: '...', body: '...'

For example, you could do this immediately before running an input step to notify a team that something is waiting.