Gfilui

I installed Postfix on my Ubuntu 16.04 server ( DigitalOcean ) ,
for my site: example.com
I added an A record mail.example.com
and an MX record for example.com handled by mail.example.com

it' running, but when I check my mail.example.com wu-ith MXToolBox , I get 1 warning

Result 
SMTP Banner Check Reverse DNS does not match SMTP Banner

Here is the /etc/postfix/main.cf

/etc/postfix/main.cf

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:$data_directory/smtpd_scache
smtp_tls_session_cache_database = btree:$data_directory/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = example.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname, example.com, example, localhost.localdomain, localhost
relayhost = 
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
home_mailbox = Maildir/
virtual_alias_maps = hash:/etc/postfix/virtual

smtpd_client_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination reject_rbl_client zen.spamhaus.org reject_rbl_client bl.spamcop.net reject_rbl_client cbl.abuseat.org reject_unknown_client permit

policyd-spf_time_limit = 3600

smtpd_recipient_restrictions =
 reject_unauth_destination,
 check_policy_service unix:private/policyd-spf

# Milter configuration
# OpenDKIM
milter_default_action = accept
# Postfix ≥ 2.6 milter_protocol = 6, Postfix ≤ 2.5 milter_protocol = 2
milter_protocol = 6
smtpd_milters = local:/opendkim/opendkim.sock
non_smtpd_milters = local:/opendkim/opendkim.sock

The error message is pretty straight forward. The banner is not matching the Reverse DNS record. You either have to update your reverse record with your hosting provider, or update your banner to match the reverse record in place.

The banner is the first line sent by the mail server when a client connects:

[~]$ nc localhost 25
220 example.com ESMTP Postfix (Debian/GNU)

It's given by the configuration line

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)

$myhostname is replaced by the hostname configured for the host, and $mail_name is replaced by the MTA in use. However you don't have to use the variables:

smtpd_banner = example.com ESMTP Postfix (Ubuntu)

will work.

To actually change the reverse DNS you have to check with your providers help desk. In the case of DO, this help article may be helpful.