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		)

		describe('mailer constructor:', () => {
		let config, mailer

		assert.equal(mailer[key], expected, `${key} was correct`)
		})
		})
		})

Contributor

This is good. But could you change the default value of sesConfigurationSet to be the empty string '', and add assert.ok('sesConfigurationSet' in mailer, 'configuration key exists') at

	'If sesConfigurationSet is not defined, then outgoing email does not contain X-SES* headers, for type ' + type,
	function () {
	mailer.mailer.sendMail = function (emailConfig) {
	var sesConfigurationSetHeader = emailConfig.headers['X-SES-CONFIGURATION-SET']
	assert.ok(! sesConfigurationSetHeader)
	var sesMessageTags = emailConfig.headers['X-SES-MESSAGE-TAGS']
	assert.ok(! sesMessageTags)
	}

	mailer[type](message) // invoke
	}
	)

	it(
	'If sesConfigurationSet is defined, then outgoing email will contain X-SES* headers, for type ' + type,
	function () {
	var savedSesConfigurationSet = mailer.sesConfigurationSet

. Maybe also assert same in the test above that?