Update, I can now send mail to external domains, but not receive from external domains.
But user1@mydomain.com & user2@mydomain.com can send and receive emails to each other.

MX record exists:
10  @   mail.mydomain.com   1 Hour
0   @   smtp.mydomain.com   1 Hour

doveconf -n

auth_mechanisms = plain login
mail_location = maildir:~/Maildir
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    special_use = Drafts
  }
  mailbox Junk {
    special_use = Junk
  }
  mailbox Sent {
    special_use = Sent
  }
  mailbox "Sent Messages" {
    special_use = Sent
  }
  mailbox Trash {
    special_use = Trash
  }
  prefix =
}
passdb {
  driver = pam
}
protocols = pop3 imap
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0660
    user = postfix
  }
}
ssl = no
userdb {
  driver = passwd
}

postconf -n

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
config_directory = /etc/postfix
home_mailbox = Maildir/
inet_interfaces = all
inet_protocols = ipv4
mailbox_size_limit = 0
mydestination = mydomain.com, ip-172-31-25-194.us-west-2.compute.internal, localhost.us-west-2.compute.internal, localhost
myhostname = mydomain.com
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
myorigin = /etc/mailname
queue_directory = /var/spool/postfix
readme_directory = no
recipient_delimiter = +
relayhost =
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes