Re: The proxy address is already being used.

Hi Frank,

Sorry for the inconvenience the issue has caused.
Could you please tell us where do you run the Disable-MailPublicFolder command? Do you run it in Office 365 cloud or On-premises Exchange Server?

Please help us do the following test:
1. Run the following command in the Exchange Online:
Get-MailPublicFolder | where {$_.EmailAddresses -match "user name"}

2. If you can get the proxy address in the Exchange Online, please modify the proxy address to another one manually.
Set-MailPublicFolder –identity username –Emailaddresses xxx@xxx.com

After doing that, try whether the proxy address has been released or not.

Thanks,
Jason Jiang