Email Delayed Deliveries

Can anyone help me try to solve these delayed deliveries?

Delivery is delayed to these recipients or groups:
John (john2@comcast.net)
Subject: update
This message hasn't been delivered yet. Delivery will continue to be attempted.
The server will keep trying to deliver this message for the next 1 days, 19 hours and 59 minutes. You'll be notified if the message can't be delivered by that time.

Diagnostic information for administrators:
Generating server: Exchange.DOMAIN.local
Receiving server: mx1.comcast.net (96.114.157.80)
john2@comcast.net
Server at mx1.comcast.net (96.114.157.80) returned '400 4.4.7 Message delayed'
11/18/2017 12:11:37 AM - Server at mx1.comcast.net (96.114.157.80) returned '451 4.4.395 Target host responded with error. -> 554 Transaction failed'
Original message headers:
Received: from Exchange.DOMAIN.local (192.168.100.203) by Exchange.DOMAIN.local
 (192.168.100.203) with Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1261.35; Fri, 17
 Nov 2017 16:21:30 -0400
Received: from Exchange.DOMAIN.local ([::1]) by Exchange.DOMAIN.local ([::1]) with
 mapi id 15.01.1261.035; Fri, 17 Nov 2017 16:21:30 -0400
sf1eldsAsked:
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
The problem seems to be in your sending infrastructure.

I just sent a test message to john2@comcast.net, relayed through MailGun, which was instantly delivered.

Either switch to using a relay service like MailGun, or go through my comments in https://www.experts-exchange.com/questions/29036679/AWS-EC2-mail-server.html + implement each item.

Using MailGun will be must faster to setup.
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sf1eldsAuthor Commented:
John2 was not the real email address.

However let me take a look at your comments.

Thanks
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RoninCommented:
Make sure you external IP which is used by Exchange to send messages to the outside world is correctly defined in the external DNS zone.
In today's standards you need to have a SPF record allowing your external IP to send on behalf of your domain. You IP also can be in the blacklist. Check your domain/server through MXtoolbox.
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sf1eldsAuthor Commented:
Not in the blacklist already checked that
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sf1eldsAuthor Commented:
Tell me more about the SPF record please
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RoninCommented:
Start with reading what SPF record is and how it's being used.

You can use the spf generator to create the record.

You would need to identify the IP address that appears on the recipient's server when your Exchange server is connecting to the deliver the message. If you don't have any special SNAT settings in your firewall for Exchange, usually it's the same IP as your normal browsing. So, from the Exchange server go to www.whatismyip.com, it will show the IP that seen by the world as outgoing connection from your network.

That is the IP you should use when you create your SPF record.
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sf1eldsAuthor Commented:
Yes I know my IP. We have a static IP issued by our ISP.
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RoninCommented:
Use the generator to create the correct SPF record which you should add to your external DNS zone of the domain.
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sf1eldsAuthor Commented:
Do I create a spf record for each domain I'm having issues with?
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RoninCommented:
Yes, indicating IP/s and A record/ FQDNs whose are allowed to send on it's behalf.
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
I've set Ronin + myself as answering this question.

Poster seems not to have followed either of our directions.

Poster seems to have stopped posting.
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