Exchange online, post migration issues

[Jonathan74]

I'm not 100% sure if this is the correct section for this, but given I have no idea what the problem is! I thought I would start here.

I do IT work for a small charity, they were running POP email accounts with only local copies of emails. Services were provided by their web provider so we had no administration control over emails.

I have migrated all emails, via PST upload to exchange online. Migration went well, all emails available and full send/receive functionality post migration.

HOWEVER! They can no longer access the website for their domain. For example: all email addresses associated with contoso.com are sending/receiving fine. On their 3 computer internal network they can no longer access www.contoso.com. It simply times out and says website unavailable.

The network consists of a BT Business Broadband router (running DHCP), a Synology DiskStation for data storage with a connected laser printer. Users log in with a local account on each computer. All computers running Windows 7.

Full connectivity to other websites works fine. The website works fine outside the network.

What I've done so far:
ping fails
Flushed DNS with IP config
Changed DNS servers to Google DNS
Host file is empty
Firewall on/off - still fails
AV on/off - still fails

What works:
I've installed a VPN chrome extension that allows it to connect

My question is... Is this just a horrible coincidence relating to the exchange migration (and I'm chasing down a never ending rabbit hole) or have I inadvertently changed something causing this to happen?

I've pretty much run out of ideas so any suggestions gratefully received.

Jonathan

[#2]

Exchange itself would have nothing to do with it, but most likely what did happen, someone changed the related DNS records for the domain when changing where the e-mail domain routes to. So your website DNS is most likely not correct. Need to find out what the IP should be and make sure it's resolving properly. Ping from local, and when it's working via VPN, see where it's pinging. If it's the same, and it works on VPN, then obviously there is some kind of IP blocking going on on the website or a device in between the site and you that is restricting certain IP's from connecting.

[#3]

@Chris Webb thank you very much for taking the time to answer, it is greatly appreciated.

I wish I could say that your suggestion helped, but I think this was one of those gremlins that has no answer... because when I checked it today it is magically working again. I have not changed anything at all, and it was simply left for 4 or 5 days while I tried to come up with a solution and it is now working.

I can only assume that there was some sort of DNS replication issue?

Either way, I now have happy users again. Thanks again.