Hello,
I face the same issue, i fixed this issue by following steps.
1. You must have valid DNS records that resolved to your Server/VPS IP and must be properly setup to your HOSTNAME. Otherwise the REBUILD POSTFIX option under email postfix > Postfix Manager > Rebuild Mail server won't fix and generate proper configuration for postfix.
2. Hostname means your VPS/Server Computer name. like SERVER1.YOURDOMAIN.COM.
3. IP address must resolve to hostname. like 1.1.1.1 pointed to SERVER1.YOURDOMAIN.COM
4. you must have rDNS entry. rDNS mean (Reverse DNS record). this will help your email reputed to hotmail/gmail/yahoomail. because if you don' have rDNS. hotmail will automatically reject your emails. gmail will get in SPAM and yahoo Blocked you.
I hope this will help others who have this issue.