Not able to send/receive emails outside my Domain from recently configured email-server

Question

Email-server setup is not working on Ubuntu 16,postfix,dovecot.
below are the config/facts

1. Registered a domain say atul.com
2. ISP is comcast who blocks port 25
3. Created A record @ -- 1.2.3.4
4. No CNAME except www
5. MX Record mail -- @
6. all ports are open like 110, 143,993,995,465,587
7. I am able to send/receive mail at local users with [email protected], [email protected], [email protected]
8. I am able to telnet mail.atul.com 465 and @587
9. I do not have any mail subscription/account @godaddy.com
10. I allowed all necessary ports on my router

---

My Doubts/Questions:

• Why I am not able to receive email from outside domain? (even when I am able to telnet on mail.atul.com 587)?
• Why I am not able to send mail to outside from my domain? while I tried setting my relayhost on below settings

relayhost = blank
relayhost = *
relayhost = email.secureserver.net

My Question on relayhost is :

• Can't I setup my independent mail server? without any intervene by comcast or godaddy? Because I don't want to use their mx records or smtp server.
• What all I need to do if I don't want to use their MX Records (smtp server)

• Please Guide me to place correct values for below Fields

   
  A Record, 
  CNAME, 
  MX Record , 
  myhostname, 
  mydestination,
  relayhost
  

Below are the Different configuration I am using...

main.cf

myhostname = mail.atul.com

mydestination = atul.com, mail.atul.com localhost.localdomain, localhost

relayhost = [email.secureserver.net]

master.cf

# ==========================================================================

# service type private unpriv chroot wakeup maxproc command + args

# (yes) (yes) (no) (never) (100)

# ==========================================================================

#smtp inet n - - - - smtpd
#smtp inet n - - - 1 postscreen
#smtpd pass - - - - - smtpd
#dnsblog unix - - - - 0 dnsblog
#tlsproxy unix - - - - 0 tlsproxy
submission inet n - - - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
-o smtpd_reject_unlisted_recipient=no
-o smtpd_client_restrictions=$mua_client_restrictions
-o smtpd_helo_restrictions=$mua_helo_restrictions
-o smtpd_sender_restrictions=$mua_sender_restrictions
-o smtpd_recipient_restrictions=
-o smtpd_relay_restrictions=permit_sasl_authenticated,reject
-o milter_macro_daemon_name=ORIGINATING
smtps inet n - - - - smtpd
-o syslog_name=postfix/smtps
-o smtpd_tls_wrappermode=yes
-o smtpd_sasl_auth_enable=yes
-o smtpd_reject_unlisted_recipient=no
-o smtpd_client_restrictions=$mua_client_restrictions
-o smtpd_helo_restrictions=$mua_helo_restrictions
-o smtpd_sender_restrictions=$mua_sender_restrictions
-o smtpd_recipient_restrictions=
-o smtpd_relay_restrictions=permit_sasl_authenticated,reject
-o milter_macro_daemon_name=ORIGINATING

10-master.conf

service imap-login {

inet_listener imap {
        #port = 0
}

inet_listener imaps {
    port = 993
    ssl = yes
 }

service lmtp {
      unix_listener /var/spool/postfix/private/dovecot-lmtp {
      mode = 0600
      user = postfix
      group = postfix
  }
}

Any Help will be highly appreciable, Thanks in Advance

More info added below

atul.com is not real domain I changed the name

nslookup
> set type=MX
> atul.com

Non-authoritative answer:
*** Can't find atul.com: No answer

and When I do 

> mail.atul.com
Server:     75.75.75.75
Address:    75.75.75.75#53

Non-authoritative answer:
mail.atul.com   mail exchanger = 10 atul.com.

Authoritative answers can be found from:

Answer 1

What you want to do is not possible. You need an open port 25.

Port 587 is the "submission" port, it is used only by end-users wishing to send e-mail through an external SMTP server.

After that, when two servers are communicating with each other to transmit a message to its destination, only port 25 is used.