import com.atlassian.jira.component.ComponentAccessor
def locale = ComponentAccessor.getLocaleManager().getLocaleFor(currentUser)
def watchers = ComponentAccessor.getWatcherManager().getWatchers(issue, locale)
def cf = ComponentAccessor.getCustomFieldManager().getCustomFieldObjectByName("Multi User Picker CF")
//get the users' email addresses, in a comma separated string, that are members of the Multi User Picker custom field and are not watchers
def recipients = issue.getCustomFieldValue(cf)?.findAll { !watchers.contains(it) }?.collect {
it.emailAddress
}?.join(",")
mail.setTo(recipients)