In this blogpost I will describe a solution for IT Administrators with the following situation:
You are unable to receive E-Mails in your Office 365 account, senders receive a NDR (non delivery report).
The NDR says:
The following organization rejected your message: CH1EHSMHS012.bigfish.com.
Diagnostic information for administrators:
Generating server: bigfish.com
martina.grom@mydomain.com
CH1EHSMHS012.bigfish.com #<CH1EHSMHS012.bigfish.com #5.4.6 smtp;554 5.4.6 Hop count exceeded - possible mail loop> #SMTP#
…this can be caused by a duplicate entry in FOPE!
To get this situation resolved you don’t need a support call with Microsoft. You can solve it with a little PowerShell magic.
First take a look in your Exchange Online Management Board where you can access the FOPE portal (if you are on an E-Plan) .
Simply click on Exchange Online / Mail control / Forefront Online Protection for Exchange.
Check if you see a duplicate entry within the Domain section in FOPE!
If that’s the case you can set the entry with PowerShell.
As I am aware that PowerShell is not everyone's most loved tool, here are all commands you have to run within PowerShell, so you can make your life easier with simply copy & paste from here.
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic –AllowRedirection
Import-PSSession $Session –AllowClobber
Set-AcceptedDomain -Identity mydomain.at -OutboundOnly $true
Set-AcceptedDomain -Identity mydomain.at -OutboundOnly $false
After that, wait for approximately one hour. Check your FOPE entries. All should be set to normal and mail-flow starts working again!
Attention: use this only if you are in the above mail loop situation. 
Hope that helps!
Martina Grom, MVP Office 365.
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