Hi @Glenn Maxwell ,
You could also run the following command to check whether your account defined in Manageby properties:
Get-DistributionGroup | ft Name, ManagedBy -AutoSize
If there is not, please run the following command to add your account.
Set-DistributionGroup SG1 -ManagedBy @{Add="<value1>", "<value2>", …} -BypassSecurityGroupManagerCheck
The following Microsoft KB article described a similar issue, you may have a look at it to know more details: (You don't have sufficient permissions) error when you try to remove or make a change to a distribution group
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