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I have done couple of Office 365 migrations and one of the basic concept of DNS Resolution and DNS Forwarders is overlooked most of the times.

Microsoft says that DNS calls should reach their closest geographical datacenter and from their they use their internal network to connect to the actual datacenter hosting tenant. It has been said that If user is making it’s DNS calls in a location on a different continent to where the user is physically located then performance would be degraded.

Let’s say during purchase of Office 365 tenant you selected location United States and all mailboxes are getting created in US based on your tenant location (US) but you have large number of your employees working from India. All these India users will also have mailboxes in US, which means they will have latency in accessing their mailboxes.

You should point DNS forwarder for Office 365 in a way that it connects to Microsoft datacenter of local geography. I did testing with different DNS forwarders and my testing shows that OpenDNS provides best round trip performance of 28ms. In this example I will be testing ping response from India for outlook.office365.com fqdn. This is primary fqdn of Outlook Connectivity. You will see that testing result varies for all of them.

It would be your choice which one you like because there are free and paid services of these.

  • 8.8.8.8 (Google DNS)

C:\Users\sourabh>ping outlook.office365.com -t

Pinging outlook.ms-acdc.office.com [52.98.71.210] with 32 bytes of data:

Reply from 52.98.71.210: bytes=32 time=86ms TTL=241

  • 9.9.9.9 (Quad9 – IBM)

C:\Users\sourabh>ping outlook.office365.com -t

Pinging outlook.ms-acdc.office.com [52.97.146.162] with 32 bytes of data:

Reply from 52.97.146.162: bytes=32 time=149ms TTL=236

  • 4.2.2.2 (Level 3 DNS)

C:\Users\sourabh>ping outlook.office365.com -t

Pinging outlook.ms-acdc.office.com [52.97.151.18] with 32 bytes of data:

Reply from 52.97.151.18: bytes=32 time=162ms TTL=241

  • 208.67.222.222 – opendns (cisco) – paid, only free for home users

C:\Users\sourabh>ping outlook.office365.com -t

Pinging outlook.ms-acdc.office.com [40.100.138.18] with 32 bytes of data:

Reply from 40.100.138.18: bytes=32 time=28ms TTL=241

I hope it has been worth your time to improve Office 365 client connectivity. Please read Microsoft’s article on client connectivity for more detail :

https://docs.microsoft.com/en-us/office365/enterprise/client-connectivity

Thank you for reading !

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