MX records. Why do I still receive email on old MX records?

A couple of weeks ago I changed our MX records to point to a new Sophos UTM filtering box that is housed on site. Mail used to be filtered by our ISP. I still have access to that filter and mail is still passed through to our Exchange server but what I'm wondering is why there is still a significant amount of email still coming through that filter for our domain.

I have the following entries:

DNS           Priority          Destination
@                10                  UTM box
@                20                  UTM box
@                30                  UTM box
@                40                  ISP's mail filter
@                50                  ISP's mail filter
@                60                  ISP's mail filter

Thanks in advance.
itmtsnAsked:
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Simon Butler (Sembee)ConsultantCommented:
MX costs mean nothing these days.

If the server is listed in the MX records, then it will get email.
Furthermore spammers specifically target the higher cost MX records.

I have a long term (now almost nine years) experiment running with a client.
Two MX records, 10 and a 20.
40% of all email comes in through the 20, but 90% of all spam attempts to deliver through the 20 host.
Both servers are available, so it isn't as if the remote site has attempted to use the lower cost.

If you don't want email coming in through the ISP, then drop them from the MX records. Then make the other three the same.

Simon.

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Frans BrouwerSenior Systems AdministratorCommented:
btw higher costs usually don't filter spam - therefore a great target for spammers - so getvrid of old higher costs MX
omarfaridCommented:
Is the DNS server maintained by you or it is maintained by 3rd party e.g. your ISP? You could simply remove those MX records which you don't want. As long as they are there you will get some emails through them.
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itmtsnAuthor Commented:
Thanks for the responses. I'm just concerned that if I kill those MX records some legitimate email won't be getting through.
Simon Butler (Sembee)ConsultantCommented:
There isn't much you can do about that.
All the time they are in the DNS, they will receive email. The only reason you wouldn't get email is because the other side has stale DNS records - again completely out of your control.

If you want all email to come through your equipment only rather than the ISP, you have to remove the ISP's servers from the records. No other choice.

Simon.
omarfaridCommented:
One note about MX records with ISP's IPs:

Some ISPs consider this as service to customers where there could be outage for the customers network or mail servers. When the customers email servers (for incoming emails) are down or not reachable then the incoming emails for the customers will be received and stored temporary on ISP's servers till the customers servers are back and reachable.
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