In some countries the IP’s are blacklisted by zen.spamhaus.org, b.barracudacentral.org. Therefore they cannot send emails to AOL, Gmail, Hotmail, Yahoo.
Other issues involve port restriction and SMTP blocking policies of ISP’s which stops IP’s to host a email server.
1.How to configure SMTP relay in YunoHost?
2.Can the relay be configured to other YunoHost servers hosted around the world?
3.Is there any other solution to bypass the blacklist?