client/lib/domains/email-forwarding/index.js
function getEligibleEmailForwardingDomain( selectedDomainName, domains = [] ) {
	const eligibleDomains = getEmailForwardingSupportedDomains( domains );
	let selectedDomain;
	if ( selectedDomainName ) {
		selectedDomain = eligibleDomains.reduce( function( selected, domain ) {
			return domain.name === selectedDomainName ? domain.name : selected;
		}, '' );
	}
	return selectedDomain || ( eligibleDomains.length && eligibleDomains[ 0 ].name ) || '';
}
function getEligibleEmailForwardingDomain( selectedDomainName, domains = [] ) {
	const eligibleDomains = getEmailForwardingSupportedDomains( domains );
	let selectedDomain;
	if ( selectedDomainName ) {
		selectedDomain = eligibleDomains.reduce( function( selected, domain ) {
			return domain.name === selectedDomainName ? domain.name : selected;
		}, '' );
	}
	return selectedDomain || ( eligibleDomains.length && eligibleDomains[ 0 ].name ) || '';
}
Retrieves the first domain that is eligible for Email Forwarding either from the current selected site or the list of domains.

@param {String} selectedDomainName - domain name for the site currently selected by the user
@param {Array} domains - list of domain objects
@returns {String} - Eligible domain name