How to send mail using external mailserver

So I finally managed to set up a Postfix+Dovecot+Roundcube mailserver on a new Ubuntu 18.04 server with the help of ServerFault and a bunch of how-to blogs online and I can send/receive email from the server now.

Now that I’m sure that the mailserver is working, what should I do to make a webserver that I already have (Ubuntu 18.04) to send emails using that mailserver? (I don’t really need to receive emails on the webserver as I can just add a subdomain for each site that points to the mailserver).

There’s articles online mentioning SMTP authentication (smtp auth to be specific), open relays, etc but I’m not sure how do I go about it and didn’t want to fiddle too much with the webserver as it has live sites and I’m worried that I might screw up.

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Mailserver Postfix Configuration:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
compatibility_level = 2
inet_interfaces = all
inet_protocols = all
mailbox_size_limit = 0
mydestination = localhost.$mydomain, localhost, localhost.localdomain
myhostname = abc.exampleDomain.com
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
myorigin = /etc/mailname
readme_directory = no
recipient_delimiter = +
relayhost =
smtp_tls_security_level = may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_use_tls = yes
smtpd_banner = $myhostname ESMTP $mail_name
smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
smtpd_sasl_auth_enable = yes
smtpd_sasl_path = private/auth
smtpd_sasl_type = dovecot
smtpd_tls_cert_file = /etc/letsencrypt/live/exampleDomain.com/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/exampleDomain.com/privkey.pem
smtpd_tls_security_level = may
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes
virtual_mailbox_domains = /etc/postfix/virtual_mailbox_domains
virtual_transport = lmtp:unix:private/dovecot-lmtp

Mailserver master.cf:

smtp      inet  n       -       y       -       -       smtpd

submission inet n       -       y       -       -       smtpd
  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject

smtps     inet  n       -       n       -       -       smtpd
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject