This shows you how to exclude shared mailboxes from a Dynamic Distribution Group in Exchange Online. We will be using Powershell to make sure your shared mailboxes don’t get spam in them.

Commands

Connect to O365 in PS:

$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session

Exclude Shared Mailboxes with this filter command:

set-DynamicDistributionGroup -Name "all@yourcompany.com" -RecipientFilter {((RecipientType -eq 'UserMailbox') -and -not (RecipientTypeDetailsValue -eq 'SharedMailbox'))}

Conclusion

Afterward, Verify in Exchange Admin Console. You always want to prevent your shared mailboxes from getting spammed in a distribution group. Consequently, I’d highly recommend setting a requirement to send to-these dynamic mailboxes. This will prevent every employee from sending emails to everyone, which will be a bad thing.