How to setup postfix on gcloud? - google-compute-engine

Question

I trying to setup a multiple domain server in gcloud instance, following this tutorial https://vorkbaard.nl/installing-a-mailserver-on-debian-8-part-3-mta-postfix/ , I modify /etc/postfix/master.cf :
smtp inet n - y - - smtpd
to
597 inet n - y - - smtpd
when I telnet domain 597 I get succefully a connection, but trying to send an email from mailx or from the telnet client I get an error as if postfix still use 25 port:
Aug 24 19:26:08 localhost postfix/smtp[1404]: connect to alt2.gmail-smtp-in.l.google.com[2607:f8b0:400c:c0f::1b]:25: Network is unreachable
So where I need to change the port to use to postfix work in google computer instance ? Telnet outside google network works ok!
thanks!
[edit]
After more debugs and test sending email to the same email send and recibe, from round cube, and from thunderbird, so I think the problem its the outside traffic from the port 25 and I dont know why postfix use 25 port if the master.cf stmp port is set to 597
mail.log debug sending from roundcube
Aug 25 00:58:59 localhost dovecot: imap(sender#domain.com): Debug: maildir++: root=/var/mail/vmail/inova.cloud/info, index=, indexpvt=, control=, inbox=/var/mail/vmail/inova.cloud/info, alt=
Aug 25 00:58:59 localhost dovecot: imap(sender#domain.com): Logged out in=50 out=511
Aug 25 00:59:22 localhost postfix/qmgr[952]: 494C582008: from=<sender#domain.com>, size=524, nrcpt=1 (queue active)
Aug 25 00:59:22 localhost postfix/qmgr[952]: 4F95180D0A: from=<nano#server.c.majestic-lodge-173213.internal>, size=461, nrcpt=1 (queue active)
Aug 25 00:59:22 localhost postfix/error[1435]: 494C582008: to=<destination#domain>, relay=none, delay=4617, delays=4617/0.06/0/0.01, dsn=4.4.1, status=deferred (delivery temporarily suspended: connect t$
Aug 25 00:59:22 localhost postfix/error[1436]: 4F95180D0A: to=<destination#domain>, relay=none, delay=80226, delays=80226/0.03/0/0.01, dsn=4.4.1, status=deferred (delivery temporarily suspended: connect$
Aug 25 00:59:59 localhost dovecot: imap-login: Debug: SSL: elliptic curve secp384r1 will be used for ECDH and ECDHE key exchanges
Aug 25 00:59:59 localhost dovecot: imap-login: Debug: SSL: elliptic curve secp384r1 will be used for ECDH and ECDHE key exchanges
Aug 25 00:59:59 localhost dovecot: auth: Debug: auth client connected (pid=1442)
Aug 25 00:59:59 localhost dovecot: auth: Debug: client in: AUTH#0111#011PLAIN#011service=imap#011secured#011session=0wbZdolXxJkAAAAAAAAAAAAAAAAAAAAB#011lip=::1#011rip=::1#011lport=143#011rport=39364#011resp=AGl$
Aug 25 00:59:59 localhost dovecot: auth-worker(1430): Debug: sql(sender#domain.com,::1,<0wbZdolXxJkAAAAAAAAAAAAAAAAAAAAB>): query: SELECT email as username, pwd AS password FROM addresses WHERE email = 'info#$
Aug 25 00:59:59 localhost dovecot: auth: Debug: client passdb out: OK#0111#011user=sender#domain.com
Aug 25 00:59:59 localhost dovecot: auth: Debug: master in: REQUEST#0112225078273#0111442#0111#0117898818d71c58f150c8d4f75bb936fb5#011session_pid=1443#011request_auth_token
Aug 25 00:59:59 localhost dovecot: auth-worker(1430): Debug: sql(sender#domain.com,::1,<0wbZdolXxJkAAAAAAAAAAAAAAAAAAAAB>): SELECT 5000 AS uid, 5000 as gid, email, '/var/mail/vmail/domain/info' AS home FROM$
Aug 25 00:59:59 localhost dovecot: auth: Debug: master userdb out: USER#0112225078273#011sender#domain.com#011uid=5000#011gid=5000#011email=sender#domain.com#011home=/var/mail/vmail/domain/info#011auth_token$
Aug 25 00:59:59 localhost dovecot: imap-login: Login: user=<sender#domain.com>, method=PLAIN, rip=::1, lip=::1, mpid=1443, secured, session=<0wbZdolXxJkAAAAAAAAAAAAAAAAAAAAB>
Aug 25 00:59:59 localhost dovecot: imap(sender#domain.com): Debug: Added userdb setting: plugin/email=sender#domain.com
Aug 25 00:59:59 localhost dovecot: imap(sender#domain.com): Debug: Effective uid=5000, gid=5000, home=/var/mail/vmail/inova.cloud/info
Aug 25 00:59:59 localhost dovecot: imap(sender#domain.com): Debug: Namespace inbox: type=private, prefix=, sep=, inbox=yes, hidden=no, list=yes, subscriptions=yes location=maildir:/var/mail/vmail/domain/info
Aug 25 00:59:59 localhost dovecot: imap(sender#domain.com): Debug: maildir++: root=/var/mail/vmail/domain/info, index=, indexpvt=, control=, inbox=/var/mail/vmail/domain/info, alt=
Aug 25 00:59:59 localhost dovecot: imap(sender#domain.com): Logged out in=50 out=511
From outside email it doesnt recive

Answer 1

Two important facts:
GCE blocks outbound email on port 25, 465 and 587 (except for port 465 or 587 to Google Apps relay only).
If you are not using a relay, you cannot choose the oubound port. This is because you must connect to whichever port your destination is listening to, which will be a standard (blocked) port.
As a result you must use a email relay, which you are not by the sound of it. At the bottom of the page I linked above are some options for email relays you can use. They all include postfix options.