Emails getting rejected from Gmail on Virtualmin/Webmin and CentOS

I have a CentOS 7 droplet that currently hosts 4 domains running Virtualmin/Webmin.

The dropled is named example.com (which is one of the 4 domains). PTR record is updated correctly with the droplet IP.

BIND DNS Server module is disabled, as I am using the “Networking > Domains” panel provided by DO. Per each of my domains, I have configured DNS as follows:

A         @       *droplet ip*
A         mail    *droplet ip*
CNAME     www     domain.com.
CNAME     *       domain.com.
MX        5       mail.domain.com.
TXT       @       "v=spf1 a mx a:domain.com ip4:*droplet ip* ?all"
NS        ns1.digitalocean.com.
NS        ns2.digitalocean.com.
NS        ns3.digitalocean.com.

Here are my /etc/hostname

server.example.com

And /etc/hosts

▽
127.0.0.1   server.example.com      server
127.0.0.1   localhost.localdomain   localhost
127.0.0.1   localhost4.localdomain4  localhost4

::1             server.example.com       server
::1             localhost.localdomain    localhost
::1             localhost6.localdomain6  localhost6

*droplet ip*    server.example.com

And /etc/resolv.conf

# Generated by NetworkManager
search example.com
nameserver 127.0.0.1
nameserver 8.8.8.8
nameserver 8.8.4.4

Finally, I have setup a forward from postmaster@domain.com to my gmail account.

Here the issue comes: if I send a test email from my outlook account to postmaster@domain.com I expect it to get it delivered to my gmail account. However, this is what I get back:

This is the mail system at host server.localdomain. 

I'm sorry to have to inform you that your message could not 
be delivered to one or more recipients. It's attached below. 

For further assistance, please send mail to postmaster. 

If you do so, please include this problem report. You can 
delete your own text from the attached returned message. 

The mail system 

<myemail@gmail.com> (expanded from <postmaster@domain.com>): host 
gmail-smtp-in.l.google.com[74.125.133.27] said: 550-5.7.1 [*droplet ip*] 
The IP address sending this message does not have a PTR 550-5.7.1 record 
setup. As a policy, Gmail does not accept messages from IPs 550-5.7.1 with 
missing PTR records. Please visit 550-5.7.1 
https://support.google.com/mail/answer/81126#authentication for more 550 
5.7.1 information. r7si49844073wmg.47 - gsmtp (in reply to end of DATA 
command) 

What am I doing wrong?