Right now if I ping "mail.mycompany.com" the resultant IP address points to our old WAN. What records do I need to create / delete / edit to correct this and allow our remote users access to email again?
So I'm assuming the "A" records are controlled by our domain host; I contacted them and asked them to point our mail server to the new static IP. Email actually works because we use a perimeter email filter and I was able to log in and redirect that myself.
No DNS records need to be changed on our local Active Directory server, correct?
For your internal DNS zone? I don't see why you would have any MX records to update. However, if you have split DNS as you design
, you would have external records defined internally as we'll so yes you would update those records accordingly.
This takes just a few seconds for a DNS admin to verify for both your internal and external DNS zones.
With regards to your email filter, it works because they likely have your email sever configured as a smart host. No MX record is used in this case. If you have email filtering service, your MX records would be pointing to their servers not yours.
This is a very common setup. I run split DNS in multiple locations and have both internal and external MX along with the helper records for exchange/lync/etc...
If the records were updated, then you must wait for the TTL to expire before DNS servers begin updating. I think you mentioned 24 hours above somewhere... New records are resolved right away if your DNS is working properly.
IF you run MS products, be sure you also updated the helpers for autodiscover, etc....