Assessing the Problem
For months, I have been battling connectivity issues with Exchange Online from home. My Outlook seems to function fine at work, on wifi tether, but when I get home, the connectivity is unstable. Some of the symptoms I’ve been seeing are:
- Message stuck in outbox
- Frequent Sync Errors
- No messages coming into inbox, then all of a sudden a stream of messages come in
Some of the sync errors I experienced look like:
17:55:58 HRESULT: 0x80040115
17:55:58 HrConfMsgAutoResolved failure
and
4:12:10 1 view(s)/form(s) added to offline folder
4:12:10 4 view(s)/form(s) updated in offline folder
4:12:10 Could not connect to public folder server.
4:12:10 [80040115-514-80040115-0]
4:12:10 Network problems are preventing connection to Microsoft Exchange.
4:12:10 Microsoft Exchange Information Store
4:12:10 For more information on this failure, click the URL below:
4:12:10 Done
My Application event log was filling up with:
Event ID 19
Rpc call (Unknown) on transport (unknown) to server (https://outlook.office365.com/mapi/emsmdb/?MailboxId={GUID}) failed with error code (80040401) after waiting (150109) ms; eeInfo (none).{{cta(‘209aad85-272d-4e83-8cdc-4f30262bc921′,’justifyright’)}}
Finding a Resolution to the Problem
Issue 1 – DNS
The first thing I did was check DNS. Opening up PowerShell, I ran:
Nslookup outlook.office365.com
I was surprised to see that my company domain was being appended, resulting in a bad IP being returned. If I typed:
Nslookup outlook.office365.com.
(Note the period at the end), the result was as expected with the list of Exchange servers. Because the resolution is occurring at my external DNS (Fios in this case), it was appending my domain DNS because of a wildcard mapping that’s telling Fios DNS servers to respond to any unknown query with a valid response. This other post dives a bit deeper into the details of the issue on how Windows is appending domain suffix to all lookups.
Fix 1 – DNS
I opened my Fios router and decided to explicitly set my DNS server to Google’s DNS servers:
DNS1: 8.8.8.8
DNS2: 8.8.4.4
After changing this, my DNS resolution behaved as expected from home. I no longer saw my work domain appending DNS queries.
Issue 2 – MTU
Although I was happy with my DNS resolution, the sync issues and Outlook symptoms continued (though only slightly better). At this point, I was positive that my DNS is pointing to correct servers and my Internet connectivity seems to be up and running. Not sure what the issue was, I relied on Wireshark to give me a little bit more information on the traffic.
Fix 2 – MTU
After opening Wireshark and looking at the packets, it was clear that there were some packets dropping from my connection to Office 365.
Opening up the Fios router one more time, I wanted to see what the MTU setting was. The Fios router sets the value to 1500. Okay, no problem. Let’s test the connectivity to Office 365 forcing MTU at 1500.
Opening up CMD or PowerShell, type the following:
ping outlook.office365.com -f -l 1500
Pinging outlook-namcentral2.office365.com [132.245.71.194] with 1500 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Interesting enough, my connection to the router is not dropping packets, but clearly the packets need to be fragmented more as it makes its way to Office 365. I kept bringing down the MTU value by 8 until I got packets through with no loss using ping to check. The final value that worked for me was 1468.
I opened my router and manually set the MTU to 1468 and done! Sync issues and Outlook are now connecting perfectly.
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