Unable to Open SMTP Port (25, 587) on Vultr

I'm using Vultr for Web Hosting and according to them they have unblocked all SMTP Ports (25, 465, 587, 2525). Now after setting up apache server and installing Postfix, Dovecot and SpamAssassin when I tested it by sending the mail I got this error: Connection could not be established. getaddrinfo for mail.domain.com failed: Temporary failure in name resolution When I run nslookup against the domain I get a valid IP but when I run it against mail.domain.com I get this error message: ** server can't find mail.domain.com: SERVFAIL I have allow both port 25 and 587 in the ufw. I checked the status of both port 25 and 587 using different technique that I found on the internet (nmap, netstat, iptables). These are the result: Does anyone know what I'm doing wrong and how can I open port 25 and 587 so that I will be able to send and receive emails. Thanks