Example shows 2 domains in 1 IP address && 1 postfix instance. All redirected to Gmail.
First domain used as myhostname
and is main domain. Others is virtual_alias_domains
Dovecot configured to use passwd database without IMAP && POP3. SMTP auth only
Gmail configured to send emails from our domains (using smtp TLS port 25).
DNS && SPF && PTR records. Example provided.
Brief config overview
Domains /etc/postfix/main.cf