Difference between sending through DNS or through ISP

When setting up Exchange 2003 one is asked whether ione wants to send through DNS or through an ISP's mailserver. It is recommended to choose DNS. I did so but soon found out that many mails were blocked because they were considered spam. Apparently an MX-record was missing. I changed back to ISP and it works now, but can someone please explain what the advantage is of DNS above ISP and what the MX record should contain?

Theo Richel
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RikketyrikConnect With a Mentor Commented:
Well you are net technically sending "through DNS" you are making a direct connection to the recipient mail server.

If I understand what you have going on. When you send mail you are relaying through your ISP and then on to the recipient. The recipient then replies back to your mail server. (I assume directly.) with no relay.

If you don't use the ISP, your mail sever directly contacts the recipient mail server. It is one less hop to make. Also, if there is some sort of problem with the ISP's mail server you will not be able to send mail. You are also subject to any relay restrictions they may have.
You will want to setup a PTR/Reverse DNS record with your ISP. Give them a call and they will help set it up for you. They need to do the work. If your mail server is using mail.contoso.com then your PTR will be that as well mapped to the outbound IP for your SMTP. (Make sure it is outbound)

Many mail servers are blocking mail without the PTR record. It is pretty standard now a days.
TheoRichelAuthor Commented:
Thanks. But that is reasoning from the view that sending through DNS is better then through the mailserver of ones ISP. I still would like to know why.
TheoRichelAuthor Commented:
OK that clarifies it, thanks
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