Postfix: external backup SMTP

Edit transport.cf and specify what to transport where, and what to discard

example.com smtp:[185.15.28.201]:25
example2.com smtp:[185.15.28.201]:25
example3.com smtp:[185.15.28.201]:25
* discard:

Edit main.cf and enter relay domains

|main.cf

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no
delay_warning_time = 1h
 
readme_directory = no
 
compatibility_level = 2
 
# TLS parameters - provide a valid certificate
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
 
 
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mail.example.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, tertiary, 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
default_transport = discard:Outgoing email disabled on this node
relay_domains = example.com, example2.com, example3.com
maximal_queue_lifetime = 5d
maximal_backoff_time = 2h
minimal_backoff_time = 15m
queue_run_delay = 15m