You may need these to configure your 3rd party antispam software/appliance.

There were lots of articles on the internet with complicated solutions, which did not work for me. Here a Powershell one-liner for Exchange management shell:

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Get-Mailbox | foreach { $_.EmailAddresses | foreach { $_.SmtpAddress } }

Within regular Powershell, add this line at the very start:

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Add-PSSnapin Microsoft.Exchange.Management.PowerShell.Admin

If you plan to run it as a batch job, the whole script could look like the following:

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Add-PSSnapin Microsoft.Exchange.Management.PowerShell.Admin
Get-Mailbox | foreach { $_.EmailAddresses | foreach { $_.SmtpAddress } } > emails.txt