Hello,
It seems your mail forwarding is currently going to a Gmail destination. Note that Google has been cracking down on spam and now require sending addresses/domain to have either an SPF or DKIM record to authenticate itself. If these records do not exist within a domain's DNS settings then Google will reject these messages. Also note SPF and DKIM records are typically for actual email addresses and not email forwarders.
Regards,
ZoneEdit Support