Configure SPF, MX and Postfix for sending email from SMTP relay

After searching for a while and reading a lot of examples, cases and/or answer on Google and Stackoverflow, I didn't find a solution for my problem. I'm more and more confused with the notion of SPF and relay, so I'm asking my question here hoping to receive a proper answer and understand once for ever !

In summary :

I have a current main server with a mailserver installed on it (postfix + dovecot + vimbadmin), on domain exampleA.com.
I have a new server for a new website, with a new domain exampleB.com.

My goal is that all the emails of the new website (XXXX[AT]exampleB.com) have to be sent & received using the main server.

Knowing that the mainserver has to send the emails of exampleB.com, I modified the SPF entry of exampleB.com to allow the main server to send emails of exampleB.com.

The mails are sent OK and I receive the emails OK, but when I look the original sources of email in Gmail :

Received-SPF: neutral (google.com: IP_MAIN_SERVER is neither permitted nor denied by best guess record for domain of root@FQDN NEW SERVER) client-ip=IP MAIN SERVER;

I'm getting more and more confused which SPF I have to modify, and which MX is pointing to who..

Edit : It seems that I have a connection timed out when my main server tries to connect to my new server on port 25, but with telnet it seems ok..

Edit2 : I know that my DNS and my Reverse DNS are not the same.. My server is an instance in a public cloud, and I can't change my reverse since it's configured in the virtual router.

Edit3 : I have this error in the main server postfix logs :

May 20 16:25:40 ns33rgdrg4 postfix/smtpd[2956]: NOQUEUE: reject_warning: RCPT from csikxdqzdqzdqzdqzdqzress.com[IP_NEW_SERV]: 450 4.1.7 <root@FQDN_NEW_SERVER>: Sender address rejected: unverified address: connect to FQDN_NEW_SERV[IP_NEW_SERV]:25: Connection timed out; from=<root@FQDN_NEW_SERV> to=<myownmail@owndomain.com> proto=ESMTP helo=<FQDN_NEW_SERV>

Here is the current configuration of servers :

Server Main

Dovecot + Postfix configured  
Domain : exampleA.com  
DNS :

 - exampleA.com.        MX    mail.exampleA.com
 - mail.exampleA.com    A     IP_MAIN_SERVER
 - exampleA.com.        SPF   "v=spf1 a mx ptr include:spf4.newsletterpartner.net ~all"

New server

Postfix installed, normally why relayhost = mail.exampleA.com

Domain : exampleB.com  
DNS :

 - exampleB.com.        MX    mail.exampleA.com
 - exampleB.com.        SPF   "v=spf1 mx:mail.exampleA.com mx:exampleA.com include:mail.exampleA.com include:exampleA.com ~all"

The MX of my new domain is pointing to the MX of my main server (I don't know if it is the good thing to do)

Here is the a part of the new server /etc/postfix/main.cf :

myhostname = FQDN_NEW_SERVER
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +

relayhost = mail.exampleA.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =

We can see that the new server has his postfix relayhost pointing to the MX of my mainserver.

I don't know if the MX of my new server has to point to himself (knowing that its own postfix has a relayhost configured), or if it has to be configured directly to the main server which will send the emails.

NB : I have this error : No SPF records found for mail.exampleA.com when I check with a website looking for MX and SPF, but exampleA.com has a MX, not mail.exampleA.com

It's driving me crazy for days now... Thanks in advance for your little helps guys :)

Regards,
Julien Q.

Solution

Ok I think it is finally solved thanks to you @henry !

In the file /etc/postfix/main.cf, myorigin was /etc/mailname, which was my FQDN.
Since Google was checking, as he says, a "record for domain of root@FQDN_NEW_SERVER", I though he had to check root@DOMAIN, and not FQDN. Google was also saying "IP_MAIN_SERVER is neither permitted nor denied by guest" so instead of putting the MX main server in my new server SPF, I put the main server IP in IPV4.

I changed myorigin from FQDN_NEW_SERVER to DOMAIN_NEW_SERVER instead, and specified my relayhost IPV4 in my new server SPF. After checking with your method AND gmail, everything seems OK now !

So here is the final configuration of my infrastructure.

Main server, Relay SMTP

Dovecot + Postfix configured  
Domain : exampleA.com  
DNS :

 - exampleA.com.        MX    mail.exampleA.com
 - mail.exampleA.com    A     IP_MAIN_SERVER
 - exampleA.com.        SPF   "v=spf1 a mx ptr include:spf4.newsletterpartner.net ~all"

New server, forwarding emails to the main server

Domain : exampleB.com  
DNS :

- exampleB.com.        MX    mail.exampleA.com
- exampleB.com.        SPF   "v=spf1 ip4:IP_MAIN_SERVER -all"

New server /etc/postfix/main.cf

myhostname = FQDN_NEW_SERVER
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
#myorigin = /etc/mailname <-- OLD CONF with FQDN_NEW_SERVER in
myorigin = exampleB.com
mydestination =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +

relayhost = mail.exampleA.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =

Gmail original sources

Received: from MX_MAIN_SERVER (FQDN_MAIN_SERVER [IP_MAIN_SERVER])
    by mx.google.com with ESMTPS id s5si18916426wme.105.2016.05.16.02.06.09
    for <FROM_EMAIL_ADDRESS>
    (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
    Mon, 16 May 2016 02:06:09 -0700 (PDT)
Received-SPF: pass (google.com: domain of RECEIVER_EMAIL_ADDRESS designates IP_MAIN_SERVER as permitted sender) client-ip=IP_MAIN_SERVER;
Authentication-Results: mx.google.com;
   spf=pass (google.com: domain of RECEIVER_EMAIL_ADDRESS designates IP_MAIN_SERVER as permitted sender) smtp.mailfrom=FROM_EMAIL_ADDRESS

Hoping it can help someone else than me.
Thanks again guys !

Julien Q.